Lots of good stuff to choose from but it really just comes down to a matter of taste and what you're going for.
But there are plenty of factors involved that cause different results:
Type of guitar being used (woods, pickups, bridge)
Wattage and amp style (amount of gain), tubes, features of the amp
other pedals in the chain
www.Pedalgeek.com has lots of different effects pedals...many of which most people don't know exist, and most of them have at least a couple of sound clips.
To my ears the Fulltone OCD is best at a reasonable price (each version of it is a bit differently voiced though)
Jumping up in price, the Keeley Time Machine is absolutely killer. The ONLY downside is the size of it--takes up some room on a pedalboard.
If you have $500 to kill, I would get the Dirty Boy Ballbuster.