Macdev 2k3 Conquest, hands down IMO. Fully balanced operation, so it doesn't need an inline reg as well, flow as good as the maxflo but it's soooo tiny -- allowing for the mounting harware, it works out lighter than a screw-in preset. Super-consistent, whether at 150psi or 650, built-in on/off and a snap to rebuild if you need to (I've had mine apart a couple of times to clean the seat, etc. but not had to replace anything yet.) The only possible mark against it is the lack of UK-based support, but since they ship everything by 48-hour Fedex, and their technical support is great, I don't lose any sleep over that. Hop over to
www.macdev.net and have a look.
p.s. No, I'm not sponsored by them, but if they're offering...
If, on the other hand, you're keeping an inline reg, get a preset. They're cheap, simple, there's not much to go wrong with them. Far less hassle in the long run than an adjustable reg that doesn't need adjusting.