Personally in my opinion I always thought that the best characters were:
The_None's Characters (Jesse, Magic Geese, Segalow and Dink Smallwood)
Dio Brando/Shadow Dio/Final Dio (Warusaki3)
Super Mario and Super Luigi (ShinRyoga and that other creator that ShinRyoga works with that I can't remember the name)
MelvanainChains' Characters (Black Glass, Boomy, Alter Amiba and Ghetto WarMachine (Then again he did collab with The_None) )
DrKelexo's Characters (Psychopath Kyo, Trouble Man, Hol Funtimes and Skaxl Roa)
Rare Akuma, Hastur and Hige (the three M.U.G.E.N jokers)
Ronald McDonald (or Donald McDonald) and Colonel Sanders
AVGN and Nostalgia Critic (mostly Duckss' version of them)
Smoke Ninja Zero and Gweelay (Brergsart)
OMEGAPSYCHO's characters (MK1 characters, MK2 characters and Motaro)
Bubsy (FourthRhyme)
and oddly enough...Sonic.EXE (Dark Chaos)