1. Senate Guildmage: As you said, the mana efficiency of the abilities make this one pretty great. Both abilities provide value regardless of the game state. The loot is of course the premier ability, but the life gain can be pretty relevant depending on the match up. So, I agree that this one is the best, but it's not a slam dunk. Some of the other guildmage abilities can provide higher impact plays, but they tend to be more expensive/board dependent and therefore are not always "on".
2. Clan Guildmage: Both abilities are pretty good for a Gruul deck. Removing their biggest blocker is often enough to let you get in for those last points, and making a land into a 4/4 can provide a much needed threat or can play defense as a plan B. Both abilities are fairly mana efficient for what you get. The biggest drawbacks with this one is it doesn't really provide much value if you drop it early, and the abilities don't provide value if the opponent has gummed up the board enough.
3. House Guildmage: The Surveil ability gets big points due to Guild synergies. Being able to trigger a Dimir Spybug or Disinformation Campaign on command is really powerful. And of course the card selection is nice, but a fair bit more expensive than Senate's. Keeping a big threat tapped down can be a big deal too.
4. Legion Guildmage: Boros is the guild that cares most about this being a 2/2 for 2, but the abilities are no joke. The tapping ability will see the most use. It's a better, but more expensive version of Clan Guildmage's red ability basically. The 6 mana ability is sort of a plan B for when you're not able to beat them quickly. It's nice to have that late game inevitability for when the bigger decks are able to stabilize.
5. League Guildmage: Drawing a card is always nice, but four mana is a pretty big investment. The spell doubling ability can be powerful, but is also rather costly and dependent on having cheap spells to copy. This guildmage is middle of the pack due to how costly its abilities are, but the power level is high if you can get to the point where it starts going off.
6. Cult Guildmage: Like League, this has a four mana card advantage ability. Unlike League, your opponent can somewhat mitigate the card advantage by hand dumping or by keeping a land or two in hand. It can be quite strong in grindy matchups, though. The ping is an ability that can always be used and is quite cheap, but can often be somewhat low impact. It does synergize nicely with the Guild mechanic and provides some inevitability.
7. Swarm Guildmage: I don't have much experience with this one, but I'd say it's a bit sub par overall. This is the first guildmage on the list I would not be particularly excited to have in my deck. The green ability is a worse version of Senate's white ability. The menace ability is quite strong if you can set it up, but it's expensive and board dependent.
8. Syndicate Guildmage: Again we have a cheaper, but weaker version of Legion's white ability. This time, though, the ability is actually useless on certain boards. The second ability is basically a worse version of Legion's other ability. It does cost one mana less, but at the point at which you're starting to dome them the extra point of damage is more likely to matter. Syndicate can be quite powerful, but it also has a decent chance of being completely useless, so it gets a pretty low spot overall.
9. Combine Guildmage: I was pretty disappointed with this card when I tried it out. Tacking on two extra mana to a creature is a big investment, and an extra +1/+1 counter just often isn't worth it. It also is a bit of a nonbo with Adapt. Moving the counters around can be nice to make combat difficult for the opponent, but overall this one just feels a bit low impact.
10. Conclave Guildmage: How the mighty have fallen. Selesnya typically gets one of the best Guildmages, but this time around we have two pretty unexciting abilities. The green ability is cheap, but often doesn't do anything. Producing 2/2's is a nice ability, but it comes at such a steep price that it often just won't do much by the time you are able to activate it.
It's kind of hard to rank the GRN/RNA ones together since they're in different environments. I think Syndicate Guildmage is pretty bad in RNA for example, the damage ability is just too inefficient (especially in the face of an Ill-Gotten Inheritance, or a Senate guildmage that can undo your work for 1 mana), and the tap ability is frequently off against everything but Gruul/Simic. Legion Guildmage is a similar card however and pretty strong in GRN which makes me think Syndicate Guildmage (a weaker version of Legion) would be decent if it were in theory part of a GRN guild (Selesnya and Golgari have plenty of big things at common while the other guilds have at least a few common/uncommon hits).
If we're ranking them in a "vacuum" I'd say Senate > House > Legion > Clan > League > Cult > Combine > Swarm > Syndicate > Conclave
Senate has cheap activations that allow you to fill out your turns when you don't curve out. The loot ability is also nice to keep the gas flowing.
House gives you a ton of inevitability with its surveil effect and the sleep ability can be useful when you're behind or in a racing situation.
Legion guildmage plays reasonably well when you're behind or ahead and provides a way to quickly win through a stalled board.
Clan is better when you're ahead than behind but the effects are cheap for what they do and can be quite powerful.
League will let you grind card advantage in the long game and the copying ability doubling a removal spell can also be relevant. However its abilities are quite inefficient compared to some of its contemporaries.
Cult's ping ability isn't exceedingly powerful but it is a bit of inevitability. The discard is a nice long term source of card advantage, however discarding a card is often worse than drawing one so I've got to put it lower than League.
Combine is not flashy, but it lets you make early drops you topdeck more relevant in the long game and can make combat difficult for the opponent
Swarm's lifegain is nice to stem the bleeding but much more awkward to fit into the curve than Senate's lifegain. The menace ability can be nice but is expensive and requires a specific situation to really be at its best.
Syndicate's tap ability being conditional is a huge hit to it but it can be good in certain matchups. 5 mana to do 2 damage is just a worse rate than 6 for 3 from Legion guildmage, but it is some amount of reach.
Conclave is frequently close to being a 1 ability guildmage, the green ability is so often irrelevant. That being said I do think there are plenty of situations where spending 6 mana for a 2/2 vigilance every turn is better than spending 5 to dome them for 2 so I think this isn't too far below Syndicate.
For reference:
Clan Guildmage
Combine Guildmage
Conclave Guildmage
Cult Guildmage
House Guildmage
League Guildmage
Legion Guildmage
Senate Guildmage
Swarm Guildmage
Syndicate Guildmage
My inclination is to put Senate highest simply for its very lost costs, but beyond that I'm not sure.
On 7/14/10, broke 1900 mark! <3 ROE.
2. Clan Guildmage: Both abilities are pretty good for a Gruul deck. Removing their biggest blocker is often enough to let you get in for those last points, and making a land into a 4/4 can provide a much needed threat or can play defense as a plan B. Both abilities are fairly mana efficient for what you get. The biggest drawbacks with this one is it doesn't really provide much value if you drop it early, and the abilities don't provide value if the opponent has gummed up the board enough.
3. House Guildmage: The Surveil ability gets big points due to Guild synergies. Being able to trigger a Dimir Spybug or Disinformation Campaign on command is really powerful. And of course the card selection is nice, but a fair bit more expensive than Senate's. Keeping a big threat tapped down can be a big deal too.
4. Legion Guildmage: Boros is the guild that cares most about this being a 2/2 for 2, but the abilities are no joke. The tapping ability will see the most use. It's a better, but more expensive version of Clan Guildmage's red ability basically. The 6 mana ability is sort of a plan B for when you're not able to beat them quickly. It's nice to have that late game inevitability for when the bigger decks are able to stabilize.
5. League Guildmage: Drawing a card is always nice, but four mana is a pretty big investment. The spell doubling ability can be powerful, but is also rather costly and dependent on having cheap spells to copy. This guildmage is middle of the pack due to how costly its abilities are, but the power level is high if you can get to the point where it starts going off.
6. Cult Guildmage: Like League, this has a four mana card advantage ability. Unlike League, your opponent can somewhat mitigate the card advantage by hand dumping or by keeping a land or two in hand. It can be quite strong in grindy matchups, though. The ping is an ability that can always be used and is quite cheap, but can often be somewhat low impact. It does synergize nicely with the Guild mechanic and provides some inevitability.
7. Swarm Guildmage: I don't have much experience with this one, but I'd say it's a bit sub par overall. This is the first guildmage on the list I would not be particularly excited to have in my deck. The green ability is a worse version of Senate's white ability. The menace ability is quite strong if you can set it up, but it's expensive and board dependent.
8. Syndicate Guildmage: Again we have a cheaper, but weaker version of Legion's white ability. This time, though, the ability is actually useless on certain boards. The second ability is basically a worse version of Legion's other ability. It does cost one mana less, but at the point at which you're starting to dome them the extra point of damage is more likely to matter. Syndicate can be quite powerful, but it also has a decent chance of being completely useless, so it gets a pretty low spot overall.
9. Combine Guildmage: I was pretty disappointed with this card when I tried it out. Tacking on two extra mana to a creature is a big investment, and an extra +1/+1 counter just often isn't worth it. It also is a bit of a nonbo with Adapt. Moving the counters around can be nice to make combat difficult for the opponent, but overall this one just feels a bit low impact.
10. Conclave Guildmage: How the mighty have fallen. Selesnya typically gets one of the best Guildmages, but this time around we have two pretty unexciting abilities. The green ability is cheap, but often doesn't do anything. Producing 2/2's is a nice ability, but it comes at such a steep price that it often just won't do much by the time you are able to activate it.
If we're ranking them in a "vacuum" I'd say Senate > House > Legion > Clan > League > Cult > Combine > Swarm > Syndicate > Conclave
Senate has cheap activations that allow you to fill out your turns when you don't curve out. The loot ability is also nice to keep the gas flowing.
House gives you a ton of inevitability with its surveil effect and the sleep ability can be useful when you're behind or in a racing situation.
Legion guildmage plays reasonably well when you're behind or ahead and provides a way to quickly win through a stalled board.
Clan is better when you're ahead than behind but the effects are cheap for what they do and can be quite powerful.
League will let you grind card advantage in the long game and the copying ability doubling a removal spell can also be relevant. However its abilities are quite inefficient compared to some of its contemporaries.
Cult's ping ability isn't exceedingly powerful but it is a bit of inevitability. The discard is a nice long term source of card advantage, however discarding a card is often worse than drawing one so I've got to put it lower than League.
Combine is not flashy, but it lets you make early drops you topdeck more relevant in the long game and can make combat difficult for the opponent
Swarm's lifegain is nice to stem the bleeding but much more awkward to fit into the curve than Senate's lifegain. The menace ability can be nice but is expensive and requires a specific situation to really be at its best.
Syndicate's tap ability being conditional is a huge hit to it but it can be good in certain matchups. 5 mana to do 2 damage is just a worse rate than 6 for 3 from Legion guildmage, but it is some amount of reach.
Conclave is frequently close to being a 1 ability guildmage, the green ability is so often irrelevant. That being said I do think there are plenty of situations where spending 6 mana for a 2/2 vigilance every turn is better than spending 5 to dome them for 2 so I think this isn't too far below Syndicate.