It depends on the knife, although I am a very strong advocate of the usage of fluoropolymer grease due to its longevity and how much it reduces friction. I believe Chris Reeve advocates the usage of a fluoro for the exact same reason given how much it will reduce wear on a knife used every day for years or decades. Additionally, this is one of the reasons the Sebenza is such a smooth-operating knife.
For knives with bronze/PB washers and most any metal-metal and metal-ceramic contact, I strongly favor a fluoropolymer grease that will resist evaporation, migration, and washing out better than any liquid product. I generally use DuPont's Extreme Fluoro and it is widely considered to be one of the finest greases on the market. Additionally, it greatly reduces actual friction and the outcome is a smoother opening and less long-term wear. Every knife I have used this on becomes much, much smoother than when factory new, plus the frequency that additional lubrication is needed is a fraction of if you are using a liquid.
When using a liquid, I generally use Eezox, but I have used TuffGlide, Benchmade Blue (which is now a dry lube), other dry lubricants, some wet lubricants, some semi-dry lubricants, some food-grade oils like mineral oil and beeswax, and actual wax. I use Eezox more when I am in a hurry or want corrosion resistance more so than actual lubrication given that kind of product does not reduce friction and wear like that of a fluoropolymer grease. Eezox is probably the best corrosion protectant made. Many products which advise corrosion resistance outright suck.
For knives that have certain internal friction parts like the Omega Springs in the AXIS lock, they get lubricated in grease as well because they are constantly rubbing against other metal and aluminum parts. I feel strongly that this reduces wear and will dramatically reduce the chance of and frequency of failure of these springs.
There are quite a few decent lubricants on the market today (and quite a few crappy ones, especially in regards to corrosion resistance). The one which I would avoid at all costs, however, is Militec-1. Many traditional wet lubricants may also be best avoided mainly because a dry lubricant and fluoropolymer greases do not tend to foul/gunk up like that of a wet lubricant, they better resist "oil migration", and they do not evaporate nearly as fast.