It's a toss up between 3 Rock Man forms for who to put in Brawl.
Rock Man (Rock Man)
Vent or Aile (Rock Man ZX)
Subaru Hoshikawa and War-Rock (Ryuusei no Rock Man)
Rock is the classic and most favored choice, he's the capcom mascot and the one we all know as "Megaman". There isn't much that needs to be said about him, he's the face of Capcom.
Vent and Aile are the most recent recreation of X and Zero style fighting with gun and sword, ZX plays like the older (Classic/X/Zero) games. Both can take the same character slot and be done by costumes in a Popo and Nana style switch (half the outfits have a girl's voice or the like). Using just the Biometal ZX moves they make for the largest movepool (if you only look at constant moves and don't include chips or stolen moves).
Subaru Hoshikawa is better known as Geo Stellar in "Megaman Star Force", he's the absolute latest Rockman in the series. If you include all moves done in the series there are 150 standard chips in the first games, most viable for moves. There are a few unique ways they could do the symbiot/host relationship between the two, but he lacks the... feel of the Classic/Z/X/ZX series, since Star Force is a sequel to the EXE series.