TBH, AC5 isn't quite the leader it once was...don't get me wrong, it's still excellent goop. Arctic Cooling MX series is pretty standard. Skinnee Labs has a thermal paste comparison
that only covers a handful, but a decent place to start. Regardless, you'll likely only see a 2-3C difference between most TIMs and that's IF you get perfect contact in testing each.
Case airflow and fans will make a bigger difference overall vs. which thermal paste to use.
BTW...I run the Q6600 as well on watercooling...I usually have it @3.4, but 3.6 at times. Great chip.