I have heard recommendations for Arctic Silver 5 and IC diamond. I also heard the warning that if you take off the lid of your CPU, IC diamond can scratch the die.
It's down to personal experience as it depends on your system and surroundings. Personally I don't mind what type I use as long as it works like thermal past not anything else.
I've just reapplied with Thermal Grizzly Kryonaut and CPU temps dropped by 2 degrees at idle and 5 degrees at 100%. Not got round to delidding yet either, so was a nice bonus.