Two of the very best are Lauer Duracoat and Brownell's Aluma-Hyde II.
Both are air curing finishes.
Lauer Duracoat is a two part epoxy that is usable in a couple of days but reaches full cure in a couple of weeks. Most users say to coat it and just leave it for a week before using.
It has to be applied with a spray device, and Lauer sells a cheap airbrush kit.
Duracoat comes in a very wide variety of colors and they also sell camouflage stencil kits.
Brownell's Aluma-Hyde II comes in spray cans and also is air curing. It's also an epoxy but not as tough as Duracoat.
It is easier to apply since it comes in a spray can, but using a spray can isn't as easy to do a good job as it is using an airbrush.
It's a tough finish with a good reputation for durability.
Other paints that people use are engine block paint, common Rustoleum oil based paint, and the various hardware store spray paints like Krylon.
Most of these paints are better if baked on, but if you allow plenty of time to fully cure they work better.
Unlike Duracoat, common paints can be easily stripped and re-done.
http://www.brownells.com/.aspx/pid=1117/Product/ALUMA_HYDE_reg__II
http://www.lauerweaponry.com/