The two best air coolers are the Noctua NH-D14 and Thermalright Silver Arrow, and the two best closed loop water coolers are the Corsair H100 and Antec Kuhler H2O 920.
Now, those are the best, but you may not need the absolute best either, so it depends on what CPU you're talking about and what OC you're looking for.
A great one that's inexpensive, yet very good is the 212 Evo. The V8 is good too, but there's not a HUGE difference between the 212 Evo and V8. The V8 looks nicer than the 212 Evo, but the 212 Evo still gets the job done quite well.