I use Akasa TIM-Clean. After years of scraping and making a mess, this stuff just dissolves all the paste after a few swipes with a cotton bud (and it smells quite nice)
I have also used nail polish remover to good effect, but as it often contains other products as well as the acetone, it can leave deposits which can affect any subsequent reapplication of thermal paste. Not a major problem but might make your temps a degree or two higher.
Make sure you wipe the surface clean with a lint free cloth, as cotton buds can leave fibres behind which again can also affect temps and surface contact of the heatsink.
Another vote for arctic silver 5!