Coolit liquid pro is the best. BUT its very nasty and very corrosive. Very dangerous on alu... actually ive seen it melt though alu. I wouldnt feel any benifit from a 3-4C decrease for that level of risk. But its all on you.
as for things less dangerous:
The best is Shin etsu X23-7738D. If you dont believe me, and i know theres a lot of die hard AS5 fans out here, google it. It beats AS5 by almost 2C. And i currently use it.
Problem with it tho is that its very hard to find, but managed to find a cheap source from a member at pcperspective. 7 dollars per bottle, min 2 orders. free shipping.
http://forums.pcper.com/showthread.php?t=415964
he has very good heatware so i feel you can trust him. its up to you.
The next after that is arctic cooling's MX-1. Again the die hard AS5 fans will flame me for this, but guys, give up. AS5 got beaten because its old. They havent changed the formula in how many years?? This beats AS5 by 1-2C variable.... But its hard as hell to apply on a naked die vs AS5. And you have to use a bit more then AS5.
After that, Yes you die hard AS5 fans can rejoice. Its AS5
So if you really nit picky about your TIM, as it doesnt improve your temp THAT much as to Lapping your IHS, click on that link, email that guy, and pick up some shin etsu X23. Remember to read on how to install it, as its a bit different. And yes it really does make a difference to prep it correctly, as to not correctly.
If you really dont care, about 2-3C difference, then use anything you have. Thats all the difference you'll really see from a cheap white ceramic based, vs shin etsu.