I highly recommend the Thermalright Ultra 120 Extreme
. I've had the aluminum version for about a year and a half now and my system has been running cool and stable with a huge overclock and voltage boost.
Make sure you get the version that comes with the LGA1366 mounting bracket. You can buy the bracket from Thermalright but its obviously easier and cheaper to just get the correct one from the start.
I've also read some very good things about these if you don't feel like spending more than $40 for a cooler: