My 2 cents is that Spoke Prep is the nicest to use as it provides a little prevention of the nipple unscrewing itself if you're wheel takes a hit of some sort and can be turned a few occasions before needing to be re-applied as opposed to LocTite types which once you've turned the nipple once the LT usually needs to be applied again. Next for me would be grease for corrosion resistance and won't gravitate to your rim tape like oil will if over applied and cheap. Oils work too but don't prevent corrosion as long as grease. Overall though it's not a huge issue with whatever you use. There are other thread lockers available which are applied and left to dry before installing the nipples and act the same as Spoke Prep but seem to be about the same price.