I use MEK -- it was the only solvent capable of taking off the stock TIM from the Rampage Formula NB. It's expensive though, cost $8+ for not even a half gallon.

Also get some of this:
http://www.amazon.com/CRC-5102-QD-El.../dp/B000BXOGNI

This is the ONLY solvent I have found that evaporates almost instantly, does not react badly with anything, and leaves ZERO deposits or streaks on heatsink bases when wiping them clean with it.