I like Prolimatech's PK-3. Its thermal conductivity rating (11.2 W/m) is higher than Arctic Cooling's MX-4 and Gelid Solution's GC-Extreme (both 8.5), which tend to be the two most popular high-performance TIMs. (And no, I don't consider Arctic Silver 5 high-performance.)
http://www.prolimatech.com/en/products/detail.asp?id=1582&page=1
Of course, there are better alternatives, but some may have serious drawbacks.
One such alternative is CooLLaboratory's Liquid Ultra or Liquid Pro. These are brush-on, conductive, liquid-metal alloys which are applied (like paint with a brush) in a very, very thin layer. Unfortunately, these must be used with nickle-plated copper heatsinks and cannot be used with aluminum or they will eat away at it. Also, because they are conductive, getting any of it on a CPU's or GPU's surrounding capacitors is likely going to cause them to fry.
Here's an application demo: https://www.youtube.com/watch?v=qFhbqiFh9Us