Qtips + 90% + alcohol does the job fine. Be very careful as if you knock anything your going to break the CPU. It's not worth the risk in all but the worst cases of thermal paste stuck on it. Which I guess is what it is. It's a slow process and needs steady hands and a clean area.
I think most electrical cleaning products including alcohol (not the drinking kind, of course) will be fine. It is, after all, just a ceramic/PCB base with gold contacts. As long as the solution will not leave a residue on there.
Angle grinder - after trying it once, you'll never have to clean the CPU, ever again. On a more serious note, TIM Clean or Isopropyl alcohol will do the trick.
Isopropyl + lint-free cloth, or just Contact Cleaner. Be sparing and don't go soaking the thing, or at least if you do give it a good few days for anything trapped under the IHS to evaporate.