You're going to get lots of, mostly subjective, responses with that question... anything from the ubiquitous Breakfree CLP, G96 CLP, and Ballistol and even Mobil 1... which a surprising number of people use.
If the idea of a product called Snake Oil appeals... there is one. "Snake Oil"
I use FP-10... mostly because I was given a near lifetime supply of the stuff a few years ago. It works fine, but I can't say objectively that it's "the best".