Anything water based or silicone based if you're using a latex condom. I have a few personal preferences for lubes, my husband has a few preferences, and everybody else who's going to answer is going to have their own preferences. Shunga's water based lube is really good, as are a good portion of the Sliquids from what I've heard. Some of the hybrid lubes are fantastic as well, they don't last quite as long as a silicone lube sometimes, but are a bit easier to clean up than a silicone based lube.