Devil's Recitation
Fucking hell I KNEW someone would say that.
Anyway, I can't come up with any particular phase but let me give ones for specific categories as to what I call good by score.
Visual AppealWinner is Giant Orb Flower from Inanimate Dream Yuuka because it looks so fucking awesome. Honorable mentions are Don't Move! from Inanimate Dream Yuuka, Gengetsu Spark, Devil's Recitation and Mugetsu's last phase.
GameplayWinner is Cheeto Lasers from Shinki. I know it's just a gimmick, but it's really fun to play against. Honorable mentions are Sleeping Terror Yuuka's Midnight Spark, Devil's Recitation, the Orriries' Sun gimmick of the SoEW Marisa battle, LLS Reimu's first phase, and pseudo-CAVE Endphase from ExAlice as a close, CLOSE second.
OriginalityDevil's Recitation. Honorable mentions are Sleeping Terror Yuuka's Midnight Spark, Inanimate Dream Yuuka's Double Spark, Inanimate Dream Yuuka's Daisy Field (Giant fucking flower; Daisy = death), ExAlice's penultimate and final phases, and Gengetsu Spark.
Edit: For the love of god we're already in the 990s? How long has this thread existed?! Since December 3rd, 2011, apparently. Don't the other threads like the rage thread last like ten seconds because I'm always showing up?