Paasche H. Seriously. Yeah, it's single action. Yeah, it's external mix. But it's bombproof, parts are cheap, and it provides the best wide coverage of any airbrush I've ever used.
Now that I'm all used to gravity feed and double action goodness, I don't think I could go back to the old H for all but the broadest painting duties (yellow wings on pre-WWII US aircraft, for example). But for priming and clear coats? Certainly.