If you’re playing on PS3 I can’t really recommend any of those games. I have a PS3 as well and while the skill levels aren’t too different, if you’re looking for really good people - the majority are on XBox, that fee means you play to win I guess. So in all of these recommendation they come with a footnote of: the community will be a little smaller.
SFxT - Fun, but the netplay isn’t forgiving of anything less than stellar connections.
UMvC3 - Frustrating to learn if you’re diving into ranked battles without any primer. Probably the most “solid” choice of this list. Personally Marvel is the most fun I can have with a fighting game, but SF4 is my favorite/better fighting game - don’t know how to reconcile those two thoughts at once.
KoF XIII - I hear the netplay is absolutely garbage, and there is nobody online. I have it for PS3 and I’ve never attempted to play online.
Skullgirls - Pretty decent online play.
MK9 - No idea, heard nothing bad - but then you’re playing MK9, which is embarrassing in it’s own right.
SCV - No idea, heard nothing bad about it.
SF4 seems to have the most “forgiving” netcode - and it looks like Marvel really borrows heavily from the same codebase, really stable connections. SFxT might be the sleeper hit that is building up steam (or it might just be Capcom Fighting Jam 2), and probably can be found for cheap now that everyone has abandoned it.
If you want to branch out into another game I’d say KoF XIII is the the more “rewarding” game to branch out to - but if you’re looking to play online I would recommend that game last, KoF is really a game you need to play in local versus to truly appreciate or level up.