Gunk engine cleaner. No joke. Helping my cousin (who is a mechanic) install a HSF a few years ago. It had not thermal grease but one of them sticky thick thermal tape jobs on it. This stuff would not come off with anything. Razor, acetone, alcohol, finger nail, etc.
Cousin grabs Gunk engine cleaner sittin in my garage, sprays it on and rubs the tape off. I proceed to clean with acetone and alcohol, and temps were fine. Been doing this for probably 5-6 years now. My temps are always good.
For thermal grease I just wipe with TP and clean it afterwards.