Arctic MX-4.. Cheap and great stuff but you want to look elsewhere for heavy overclocking. I buy this stuff in bulk.
Noctua NT-H1...Better for Overclocking overall really good.
Arctic Silver 5 99.9% silver.... It's a little better than NT-H1 but you need to be careful applying it, don't over do it and it takes a few hours to settle in to give you the best performance so you have to wait a bit to stress test.
Thermal Grizzly Kryonaut.. 1 gram= $12. Expensive but it's the best you will find in the Ceramic/Carbon-Based range.