Dead or Alive Dimensions (3DS) is probably the best portable fighter there is. It can be had for 15-20 retail and is a damn bargain at that price. Here's why it's good: No one wants (or should) play a serious fighter on their handheld. That shit is reserved for joystick jockeys and lag free environments. DOA has a serviceable albeit hilarious story mode and the fighting is silky smooth and quick. Don't bother with the 3D slider, it looks like ass.
DOA 5+ for the Vita might beat it out in the Spring, but Dimensions is really my go to game for handheld fighting. SFxTekken and BlazBlue on vita are good choices, but a little too twitchy for handheld gaming.
If Tekken is your thing, there's polished version of the Tekken 6 engine on the 3DS called Tekken Prime. I think it can also be had <$20. Enjoy!