I like KOF XII a lot. It is a truly great fighter and it is extremely competitive. I can't comment on BlazBlue because I have not played it. Go with KOF because well... it's the King of all Fighters.

I'm siding with BlazBlue, only because it's wildly different than both KOF and Street Fighter. I think the characters and art look way better in it, the effects in KOF with movie maker explosions look pretty bad. But that's just me.

I'll probably get both somewhere down the line here, but BlazBlue would be first.

Never liked Guilty Gear.  Especially the whole foreground/background fighting nonsense. 

I'd say KOF. While I do like the art style of Blazblue and the Guilty Gear games the backgrounds and characters seem to blend together and make it difficult to tell whats actually going on. Maybe it'll be different when I'm actually playing it and have it on a nice screen but as it stands right now KOF just looks fantastic and has a nice history for being awesome.

For me I say KOF, but thats cause I really don't like the look of Blazblue.  The characters look like a paper cut out layed ontop of the 3D back grounds.  The characters look like they have no weight in their environment and that gives the game a disjointed feel.

I want to get another fighting game. I've been rocking SF4 since its release, but I need something to switch off with.