Personally I say that it depends on what kind of games you are playing. If you're going to play games where analog triggers are beneficial (racing games for throttle threshold for example) then I like the xbox360 controller. If you're going to be playing action games where L2 and R2 don't have variable functions (either you press them or you don't) then I actually like the Playstation 2 Dualshock (since it has L2 and R2 buttons instead of analog triggers) with a mayflash PS2 to PC adapter. If you're going to be playing fighting games, I personally like fight sticks from Hori and Madcatz that use Sanwa buttons and sticks, or if you play on pad I think that the originally SEGA Saturn2 (Saturn JP) 8 button controller is the best.
Not a straight forward answer, I know. I am a big proponent of using the right tool for the job though, and different games benefit from different input devices. It's one of the benefits of being a PC gamers. So many options. Look at what you play, and pick your poison from there imo.