asus maximus formula hands down.
it has so many extra little features that I have found very useful. just little details like the lcd poster which tells you in english what is wrong, hdd activity leds on the board, voltminders leds on the board, halfway decent cooling, onboard audio, a damned near bulletproof BIOS.
I have always been disappointed with various boards I have bought in some way, but so far I am very pleased with the maximus.
the p5 is practically the same thing I think (it even says maximus formula under the sticker i believe, but stripped of all the cool litle features. If you're going to buy a board, get the best one you can afford.