Tuniq TX-2 [silicon oxide grease]
This is what I use.
OCZ Freeze [aluminum oxide]
Arctic Cooling MX-2 [aluminum oxide]
Noctua NT-H1 [silicon oxide grease]
I’ve seen this one recommended for those who have a phase-change cooling solution, as the Tuniq stuff apparently will not work with it.
Arctic Silver 5 [polysynthetic silver]
The disadvantage of this one is the 200-hour curing period necessary before it will perform at its best (however, they say that even after the curing period, its performance isn’t any different). I used to use this one.