The biggest name in thermal compounds (aka TIMs or thermal interface materials) is Arctic Silver. They have been at it for a long time and are the most respected company in the game. Currently their two most popular products are Ceramique and, at the higher-end, Arctic Silver 5. The "Arctic Silver" line has been around for some time- it is the companies flagship thermal compound and is usually regarded at the industry's best. Note that the previous compound was AS3, as no AS4 was ever produced.Read more
XYZ Computing has posted a review of Arctic Silver Thermal Compounds- AS 5 and Ceramique.