Best power wise, or price wise. The most powerful card out there at the moment is the GeForce GTX 280. (About $450.) I doubt you'll actually need one of those unless you benchmark/fold/or you're an extreme gamer. :>
What is best is a matter of perspective, what you need it for and your budget. What is best on the market today is old news tomorrow. If you assume you need a better GFX anything that was released after your current one is the best.
Generally Nvidia from what I hear. My ATI just kicks Ted's Nvidia which is like a brick in his comp. Try to defend it Ted. VERTEX ERROR