Might as well chime in on the 4 options Ciato noted as tenative:
MvC2: While not what I'd call "Traditional" works well enough for a 2D fighter. Its different than the usual 2D Fighters, but easy enough to pick up, and the usual flaws don't really apply to most of the DL proper. Also, worth noting that MvC2 is actually a pretty good spectator game, since matches are insane and hilarious shit happens.
Smash (be it Brawl or Melee): Fine, but again, I stand by we should at least do SOMETHING different than the usual "Tourny rules only!" play. Items (to whatever degree people find reasonable), teams, Free For All, what have you, just NOT 1v1, NO ITEMS, FINAL DESTINATION ONLY! like we always do (ok, we did fight on...the Star Fox stage last year, I think? But that's still kind of boring, limiting to one stage.) We can work out the specifics later.
Tekken: Works as well as anything for a 3D Fighting game. We'd done Soul Calibur twice (SC3 at DLC2, SC4 at DLC4), and Virtua Fighter once as well, Tekken probably is best on grounds of "We haven't done this before, and its a mainstream title"
Melty Blood: Um...er...I dunno, I know practically nothing about it other than there's a few DL members that seem to have a cult following within it. Though I should note one thing:
People who support Melty Blood have NO RIGHT to talk about Unfair Advantages regarding How Played a game is (something used against MvC2 at least, compared to TvC), since Melty has a pretty big gap in playership, as we have those who play it relatively regularly, and those who have never even touched the game and know practically zilch about it.
Not saying "DON'T PUT IN MELTY!"; I'm undecided where I stand on that cause having never seen Melty AT ALL (so as far as I know, the game is a myth!), I can't actually make a serious opinion on the game here.
I'm saying just saying "if you support Melty, then don't try to pull a skill gap argument."