I'm gonna make this list based on buster only situations:
MM1: Cut Man - jumping and throwing Rolling Cutter at random is really annoying.
MM2: Quick Man - same as Cut Man, but faster.
MM3: None, I can buster them all with relatively no problems. Shadow and Needle tend be tricky sometimes, though.
MM4: Pharaoh Man, for sure. Can't really beat him without Flash Stopper yet.
MM5: ...I don't remember using the buster for anything other than Charge Man, so I guess Stone Man would be tough with buster only.
MM6: Tomahawk Man, even with his weakness! How ironic.
MM7: Turbo Man and his annoying fire damage attacks. I go for him first when using the 8 RM start code, though.
MM8: Search Man. So much Search Man.
MM&B: Fucking Dynamo Man.
MMGB1: Elec Man, because his pattern is unpredictable and there's no boxes to stay on top of unlike the NES battle.
Haven't played 9, 10 or the GB games from 2 through 5.