The best? As many answers to that as people you ask (sort of). IMO, with a stock engine like what's in the Sport Trac, it probably doesn't matter much. I'd say, go with a stock replacement (Motorcraft).
The real question, to me, is, how much of your life do you want to spend replacing them? It's not brain surgery, but it is a PITA. Might put your time above your dollars and pay to have it done.
