I would look for the following:
* Doesn't transition from smooth (good) to sticky (bad) over the course of a long ride
* Easy to apply before a ride, and during if needed
* Reasonably easy to wash off from both skin and shorts
* Hypoallergenic if possible. Even if you're not initially allergic to the ingredients, it's possible to develop something after enough use -- happened to me with sunscreen during a summer of touring.
I've been using BodyGlide for randonneuring for about 3 years now and it meets all of my criteria. It comes in a deodorant stick format which is easy to apply and doesn't seem to break down even on 600km rides (although it does need re-application every 200-300km). The only problem is it doesn't ever completely wash out of the chamois, but that almost seems like a plus.