Skullgirls is a lot of fun but there's so few playing it that most of the time the matchmaking code just goes ahead and pairs me with a FGC pro who probably has a $400 fight stick and double-perfects me.
But on those times when I get someone who's actually near my level enough that it's competitive, it's a blast. And such amazing artwork. 2D is not dead.
I typically see Capcom vs. SNK2 within the top 3 of most competent player's lists. You get Mai, Maki, and Chun li all in one, what else do you need scrubs?
Competitively, Street Fighter II (ST, HDR, or TFC). I had more actual fun with Dragon Ball Z: Budokai 2 playing with my friends though (single player was cool too).
I want to give an honorable shout out to Killer Instinct (2013). That game is chaotic and visceral in the best way possible. If fighting an Arbiter from Halo as Rash from Battle Toads sounds interesting to you, then KI2013 is right up your alley.