Which thermal compound to use

I can second for the diamond, it's some pretty good stuff. I got a 4 gram package for like 7$ on EBay. Got it a year ago, I've done like 6 computers so far and still have a lot left. I've used it on my video cards too and had good results. I have Formula 6, not sure if they're still on that or not, but it's a good option.