AMD's best CPU is not a single CPU. You have get two of them. AthlonFX-74 for QuadFX setup. (nobody can afford them, and even if one could, he's go for Intel) They are releasing a 3GHz AthlonFX for normal socket AM2 systems shortly. Until then, AthonFX 62 is the fastest.
If you do not do lots of multiprocessing, Core 2 Extreme x6800 is the fastest Intel CPU. Otherwise, Core 2 Quad Q6700 is the fastest. But it can't be well utilized with present software. So it's a waste of money to buy one right now.