what is the best thermal grease or paste

I think that Indigo Xtreme still holds the mantle of being the best TIM. However, that has a strange installation procedure, and isn't for everyone.

Arctic Cooling MX-4 is cheap, widely available product that has no cure time and isn't electrically capacitive/conductive. I always recommend it, because paying upwards of twice as much for just a couple degrees - if that - of performance doesn't make much sense.