One of the best, if not the best, is wax-oyle. I've been behind it in vehicles that were totally rotted out, except where this stuff was applied. Those areas were still perfect. It's a waxy oily mess, as the name implies. There are home made versions of it consisting of bees wax, cooking oil and the like. I don't know if they work as well as the official stuff does, but I suspect they would.

POR-15 is up to you. I've had it fail badly and quickly in all applications, so I'm not at all impressed by the stuff. But others seem to have had good results with it. In my experience, Rustoleum rusty metal paints held up better then POR-15 does.