I have two favorites; one for artwork and the other for functionality/flavor text.
Dreamborn Muse is by far my favorite in terms of artwork. It doesn't have anything fancy about it, just the simplicity in design but it is still is eye catching. Still working on getting my 4th Onslaught foil and 4 10th foils.
Mizzium Transreliquat is one of the most versatile cards in the game, and also one of the more difficult for other people to pronounce. "What is it? Um...What do you want it to be?". Extremely true, as this card with Time Vault allows for unlimited turns for 3 colorless mana, or for a boatload of mana, change it to Panoptic Mirror and imprint whatever instant you want then change it to Isochron Scepter and pelt a high cost instant for 2 after that whenever you make it a stick, or, one more convoluted example, Mimic vat, kill whatever you want, then swap to soul foundry and mass produce permanent copies. I'm still working on my foil/signed collection on this as John Avon actually does signings for his cards (1 foil, 1 signed, and 1 foil/signed)
4 Angel of Despair
2 Razia, Boros Archangel
2 Memnarch
2 Arcanis the Omnipotent
2 Seedborn Muse
4 Eternal Witness
1 Blazing Archon
Artifacts:
3 Mizzium Transreliquat
2 Sensei's Diving Top
4 Dragon Arch
4 Quicksilver Amulet
4 Isochron Septer
4 Crystal Shard
4 Mycosynth Lattice
2 Darksteel Forge
4 Sculpting Steel
1 Door to Nothingness
4 Boomerang
Land:
4 Cloudpost
4 Urza's Tower
4 Urza's Power Plant
4 Urza's Mine
1 Tolarian Academy
4 Gilded Lotus
3 Elvish Piper
2 Privileged Position
2 Platinum Angel
1 Tooth and Nail
1 Door to Nothingness
2 Sylvan Scrying
Theoretically, this should be able to drop an Angel of Despair 3rd turn, with a Boomerang Isocron on the field, Dragon Arch, and a set of Urza lands. If there is something that would work better with that concept please leave a comment about it.
2 Razia, Boros Archangel
2 Memnarch
2 Arcanis the Omnipotent
2 Seedborn Muse
4 Eternal Witness
1 Blazing Archon
4 Mizzium Transreliquat
2 Sensei's Diving Top
4 Dragon Arch
3 Gilded Lotus
4 Isochron Septer
4 Crystal Shard
4 Mycosynth Lattice
2 Darksteel Forge
4 Sculpting Steel
1 Door to Nothingness
4 Cloudpost
4 Urza's Tower
4 Urza's Power Plant
4 Urza's Mine
4 Forest
3 Elvish Piper
2 Privileged Position
2 Platinum Angel
1 Blazing Archon
1 Door to Nothingness
2 Worship
Theoretically, this should be able to drop an Angel of Despair 3rd turn, with a Boomerang Isocron on the field, Dragon Arch, and a set of Urza lands. If there is something that would work better with that concept please leave a comment about it.