Krytox is spendy as heck, but it works very well. I lube all the lights I mod with 50/50 and I've yet to hear a complaint.
A few other known-to-work-well lubes are:
Nyogel (available at lighthound)
Pure Silicone grease
Magnalube-G
It's a topic that has been discussed to death. I listed a few of the most popular here, but I guarantee that there's 100 more other people might suggest.