I guess there's at least one RM per each game that really got the short end of the stick.
MM1: Bomb Man. Definitely. Everyone remember Cut, Guts and the elementals... but no love for Bomb.
MM2: I would say Bubble Man, but at least he featured on some relatively big games, so... Flash Man.
MM3: Top Man. No-brainer. (also, when I watched Mega Man on TV, I thought he were very cool and cunning. The guy even charmed Roll.)
MM4: Hmm. It's almost a tie with Drill Man, Ring Man and Bright Man, but Drill is still the most forgotten. (Also, I liked Bright Man on the TV series, his power were cool.)
MM5: Star Man. In fact, he was the only RM I completely forgot of MM5 before I played MM8BDM... Charge is a close second.
MM6: Let's see... Wind Man. I have yet to see one person to say he is one of his favourites.
MM7: Burst Man. He was supposed to look like... A Japanese bubble-blower mixed with an Edo Castle? Eh. But Cloud Man is almost forgotten, too.
MM8: What to say? Aqua Man and Search Man. Not many fans of these.
MM&B: Pirate Man. He doesn't put up a good fight at all. Also, Cold Man.
MM9: Jewel or Plug. Can't decide...
MM10: Commando isn't memorable... unless you can remember his hellish stage. SANDSTORM! D:
Also, ALL the Stardroids.