RM&F2: Compass Man. I have not managed to beat him. He seems invincible to every attack. I'm gonna guess there's some MM2 Alien-type stuff going on here
His weakness is, somewhat counter-intuitively, his legs.
I'm gonna copy your format now!
MM1: Ice Man. Just don't like him as much as the rest, and he's about as bad to play as as Oil Man in Powered Up.
MM2: Quick Man. Don't like his weapon, design, stage, or boss battle.
MM3: Shadow Man. Just not as big a fan as I am for the others.
MM4: Dive Man. Same reasoning as Shadow Man above.
MM5: Charge Man. Stinks. Terrible design, terrible stage, terrible weapon, terrible battle.
MM6: Wind Man. He's not very aerodynamic for a Robot Master that's always flying around, I find that ridiculous.
MM7: Turbo Man. Hate the stage, hate the weapon, hate the battle.
MM8: Grenade Man. I just find all of the others much more interesting.
MM9: Does Fake Man count? If not, Plug Man, just because I don't particularly like him, unlike the rest.
MM10: Nitro Man. Just kinda boring.
MM&B: Magic Man. I'm meh on him, but like the other 5 originals.
MMV: Uranus. I don't like how tough it is to use his weakness against him. He's still pitifully easy, but not any fun.
MMI-IV: Enker. I like Quint in general, and I enjoyed fighting Punk and Ballade. I find Enker boring.
Wily Tower: Buster Rod G. I still like him, I just think his fight isn't as interesting as the other two's.
R&F2: Komuso Man. I don't really get his design, and I felt he was the hardest boss in the whole game.
DOS: Volt Man. Worst stage, worst battle.
DOS 3: Bit Man. Despise that design.