I am currently using Dow Corning TC-5022 Thermal Compound. When I bought my first aftermarket cooler, it came with a bit of TC-5121, which is a thermal compound of a cheaper grade for economical applications. I had no clue if it was actually good or not, so I did some research, as well as some tests. Dow Corning is a respected company and makes all sorts of silicon products; their thermal compounds are used mostly for industrial purposes so there is no marketing or "fluff" behind the numbers they provide you. I made the decision to stick with their products - after all, TC-5121 managed to give me the same results as Arctic Silver 5 and Arctic Cooling MX-4.
....buuuuuut Dow Corning products are kind of hard to get a hold of in the commercial sector. Since I was so interested in experimenting with things, I had my tube of TC-5022 ordered and shipped from some place in Taiwan for about $18, which is more than what anyone should realistically pay for any thermal compound. (Including Indigo Xtreme, which I think still holds the crown, in terms of performance?)
All this said, I always recommend Arctic Cooling MX-4 to others. It's affordable, it's widely available, and works great. Anything that works appreciably better generally isn't worth the extra cost you will pay for that small performance gain. On a minor note, it is also very forgiving; people often add too much thermal compound, MX-4 actually seems to prefer that, and how you apply doesn't seem to make a world of difference either. It's just a good, well-rounded product that you can't go wrong with.