Tekken. I have yet to find a fighting game franchise that compares to it's quality, and not without a lack of trying. I even got Virtua Fighter 5 in hopes that maybe I'd like it just a bit more, but it just comes off as slow and extremely unpolished (and believe me, I really wanted to like it).
Tekken is just so fast-paced, smooth, and exciting. More importantly, a lot of the characters actually FEEL different rather than just slightly modded versions of the same dood (Steve Fox is an excellent example of this). Yeah, there are characters that are a little too similiar to each other, and I would like to see more diversity in the next proper sequel. Plus, the controls are just beautiful (not perfect, but the best I've seen). A button for each limb? Aswesome. Another thing is that I prefer the more down-to-earth (robots and fighting animals, not with withstanding) titles. That and it pulls off the whole "easy to learn, difficult to master" very well.
Smash Bros. is probably a distant second just because it's so unique.
I've been meaning to pick up a DoA game for the longest time but I don't own an Xbox. The only game I've played in the series was DoA2 demo for the Dreamcast yonks ago, and I really liked it. I don't remember the controls being as good as Tekkens, but it's just as fast-paced (maybe more so) anbd the levels were WAY better. If I ever get a 3DS, I'll definately pick up Dimensions (or if somehow get an xbox I'll get 4).
I've also been meaning to pick up a new Soul Calibur game, and I've been wondering if I should get 4, or just wait for 5. I have two, and it was pretty good and they did put effort in the single player which I do applaud. I just can't find a whole lot of characters I like so I just stick to Nightmare or Siegfried or whatever (which I know I shouldn't do). So if any of the fans read this, I'd love to hear your opinion.
PS I have played Super Street Fighter 4, Mortal Kombat Deception and Armegeddon, and Marvel vs. Capcom game (I don't know which one) and none of them appealed to me. I tried BlazBlue and I think I might give it another shot.