OCZ freeze is my favourite, no messing around just blob and fit. Performance is better than MX-2 in my experience (1 to 2C) and its unfussy nature reduces the chance of bad mounts.
For best performance I'd chose, and use, Coollaboratory Liquid Pro. It's a bit fussy, surfaces must be clean, and it can be a pain to remove. Get it right, a blob 1/2 the size of a small grain of rice is enough for an ihs and the base of a cooler, and it will easily make the most of your lapped set up beating OCZ freeze by the customary 1 to 2C.