Well now....my least favorite RM...though choice considering I like all of them, and I haven't played the games in a while. But if I'd have to choose, they would be the following RMs:
MM1-BombMan- I like all the original RMs and their designs, But Bomber would be my least favorite for being one of the easiest bosses to beat (true cutman is even weaker than Bombman, but I can't really hate, much less dislike the little guy, or his memorable theme)
MM2-FlashMan- For being a incredibly easy boss, even more than BubbleMan
MM3-TopMan- Weak ass boss with cheap ass weapon
MM4-ToadMan- Because I just don't like the bot...grrrsh-ness
MM5- Though choice, half of the bosses here Are my least favorite RM bosses of the entire franchise in my opinion. But I do know who desserves the title of most hated...ChargeMan, cheap as boss (not the though cheap, I mean the ohmygodthisguyissocrappy kind of cheap) and with an even cheaper Master Weapon.
MM6-WindyMan or whatever his name is. Airman wannabe. U CAN'T COPY AIRMAN *cue Airman can not be Defeated theme*
MM7-I'm gonna have to go with 2 robot Masters here, officialy making this Rockman game, have the least likable bosses in the franchise. BurstMan-for being too easy (aside from CloudMan and the rest of the bosses) and of course SlashMan- Wolvering, why are you in Jurrasic Park anyway? What do you have to do with dinosaurs? And what in the name of "Ductuh Wahwee" are you doing in the ceiling thats secreting red sticky liquid? You know what I don't wanna know. (actually I do) Anyway your here because I thought I liked you but then I realized I really didn't...also your cheap (though cheap this time)
MM8-Well I can't really hate ClownMan (cause I like clowns...especially evil ones, course this clown is just gender confused), can't hate FrostMan cause he reminds me of Frosty on steroids, can't hate TenguMan cause I love his design, SearchMan because his the first 2 headed RM, SwordMan cause swords+fire=awesome...unfourtinately this doesn't apply to SM. As for the last 2, AquaMan is just...well I don't know, just like ToadMan I can't seem to like him...but I can't seem to hate him either. And GrenadeMan for being quite easy...and the fact that dieying (please correct if written wrong) made him...feel good. *shrugs*
MM&B-Dynamo for a obvious reason, a reason that everyone else has already explained
MM9-Hmmmm Pluggy or Magmour...eh I'll choose both. Pluggy for his magnetic shockballs of doom, and Magmour for being predictable (though he still does kill me from time to time (at leats when doing a p-shooter run))
Ands thats pretty much the only original Rockman games that I've played.