All of them. At the same time. That's what Scramble Battle should have been.
Speaking of which, that was canceled, correct?
What the hell were they smoking when they canceled it? There's no licensing issues to deal with, a pre-made battle system that would easily be exploitable and usable (pick Smash or Jump Stars, either were great multiplayer fighting games), and a [parasitic bomb]-ton of characters to choose from. Pick like 2 from each series that are of different 'types' to avoid conflicting ones (like if they added Fire man, they shouldn't add Heat Man or Flame Man, and so on), or at least not have like a crap ton of the same (I wouldn't mind Fire Man, Flame Mammoth, or HeatMan.EXE, or even Taurus Flame ALL being in there, it's at least a character from each series). Heck, they could just have the fan favorites as fighters, and everyone else as support characters. Jump Stars did that, and they even pulled it off in Marvel VS Capcom 1, and both times it was awesome.