Oh. Hey. This topic is alive again. Guess I should update.
MM1: I've learned to handle Ice a while ago; the only guy who still gives me trouble sometimes is Fire-Man. Weaknesses or no, he is in general a pain to fight.
MM2: Unchanged from Crash-Man. Quick-Man probably deserves the spot more, but he's never given me as much trouble as good old Crash did.
MM3: Unchanged from Shadow-Man. It's been a long time since I've actually played MM3, and even though I've learned some of his tricks, he seems like more of a pain than the other RMs.
MM4: I was probably overly frustrated at Dive-Man at the time of writing this because he killed me so much. Ring-Man is probably the real pain here; I remembered beating him using his weakness and a small stack of E-Tanks the first time.
MM5: Probably Wave-Man or Crystal-Man. Everyone else's pattern in the game, I have down to a science, but Wave-Man can throw a wrench into that with his random Water Wave, and Crystal-Man has a large number of projectiles going for him.
MM6: I really can't remember a lot about my MM6 experience, but I vaguely recall having trouble with Blizzard-Man. Might have been his stage, though.
7FC: Turbo-Man. It was honestly a lot of stupid deaths, but this guy killed me more than any other 7FC RM. The timing on his motor-car attack was really the only big thorn in my side.
8FC: Search-Man. Homing missiles in excess along with a hide-in-the-bushes gimmick was more than enough to make me dislike this guy, but then they threw in the scopes. Dodging this guy has been one of my greatest challenges as a Mega Man gamer.
MM9: I don't even know why I said Concrete-Man the first time. Even with his weakness and a small stack of E-Tanks, I had trouble beating Plug-Man. That guy is just a pain.
MM10: No discredit to Chill-Man—I still remember all those kills you got on me in the boss room, bud—but after some reviewing, Solar-Man seems more difficult. Similar amounts of damage, similar amounts of trickiness, but in Solar-Man, the boss room is also against you—the varied elevation makes for much more annoyance than ice physics.
RNC: Clash-Man. Wow, what a shocker. He was basically a souped-up version of the Crash-Man I knew, with his new explosive detonating the entire floor instead of just one spot. Failing to time my jump properly quite often, I met many an end against this foe.
4MI:
Shadow-Man Dust-Man was probably the trickiest RM here. His randomized Recycle Inhaler made dodging a pain, and there were more than a couple embarrassing deaths where I was just sucked into him, turned into a -Tank, and kicked into a furnace. I didn't get as mad at Dust-Man MI as I did at other tricky RMs, though—the mark of a really good boss, if you ask me.
Rokko Chan:
Fransiska & Lancea Rolling-Man. Rolling is a lot like Crash in the sense that I seem to be the only person on the planet who has trouble with him. I dunno. I guess it's the projectile spam that gets me. At least I can kill basically every other RM in the game with little to no trouble.
Christmas Carol: Future. Anyone who's ever tried to Dragon him will know exactly why Future.
Christmas Carol 2: If we speak of Normal Mode, it's Want. Want was the only one who really gave me any trouble on my first playthrough—Scrooge had some windup to all of his attacks, and Ignorance and Tim were basically "gimmick" RMs where once you learned how to hurt them, killing them was a breeze. On Hard Mode, though, Scrooge gave me a lot more trouble because he kept healing himself, making him the hardest.
MMU: Glue-Man. Just...Glue-Man. I could probably go start an entire topic about Glue-Man's bullshit. To those of you who are content with that knowledge, advance. To those of you that wish to know why, enter the spoiler below.
Why not? This guy's gimmick is basically half-piping back and forth across the boss room, firing a Glue Shot down at you every time he reaches one side. He has perfect aim, and the boss room layout is designed in such a way that outmaneuvering a shot is frustratingly difficult, and if he hits you, it's a basic guarantee that he will collide with you and deal 8 contact damage. ...Yes. 8 damage. Of the many poor decisions MegaPhilX made in this game design, giving all of his RMs 8 contact damage was one of the worst. So basically, since Glue Shot can't hurt you, this RM's basic damage method kills in 4 hits, and is more painful to dodge than Thunder Beam or Ice Slasher ever was. Did I mention that he's half-piping back and forth really quickly, and might crash into you without landing a Glue Shot in the first place? His weakness does half the damage of his basic method of attack and is harder to hit with; this should never be so! If there's one thing MMU taught me, it's to never make a boss fight like Glue-Man's.
MMRevo: I didn't find any MMRevo RMs particularly difficult—except for Storm-Man. He stays out of range enough and his homing Storm Chaser does enough damage to make his peskiness notable. Beat and some other weapons help ease the pain, though.
MMRF: I think Photon-Man was the only Robot Master who ever actually killed me. So I guess he wins by default.
Mega Pony: Loyal Mare. Mega Pony had its fair share of tricky boss fights (Though Magic Mare was unexpectedly simple and easy), but Loyal's struck me as blatantly unfair the first time I tried it. Darting around her boss room and jumping randomly, she goes into her Sonic Rainboom charge attack whenever she nears, and...well, she just does a fair amount of damage. She was basically Quick-Man, only exchanging projectiles for more damage. She was very pesky, and it took me a long time to decipher a pattern for her.
Rockmen R: Power-Man. Rockmen R quickly sets up a precedent that hitting bosses with charged shots knocks them back in a manner similar to the way you do any time you get hit. Then
this guy marches into the room and smashes that precedent into a million pieces. Being the largest RM in the game, jumping over his charging attack is difficult (Especially with RMR's wonky physics), and it does ludicrous amounts of damage on contacts. If you're not a fan of
that attack, his other one involves him pounding the ground and sending off a bunch of randomly arcing rocks, which are equally annoying to dodge but do less damage. His weakness is the most clunky weapon ever, too—imagine Hyper Bomb with a much longer fuse and an explosion radius that's not even taller than Roll. His only saving grace is that said weakness kills him in a sweet two hits.