The absolutely best cable you can get, is called wet noodle cable, or just silicone wire. It's meant for RC cars etc, and has very fine strands, making it very flexible, while the combination of that and the silicone insulation, give you the advantage of being able to shape the wire, and it'll stay like that. It's also very hard to break. It's simply made for more heavy duty use than you'll ever see in an AEG. It's not too expensive either. Check around at the RC stores on the web. 18 AWG should be more than enough, and 16AWG is what manufacturers like Intellect use on their batteries. If you go for 16 AWG you might get into trouble, at least with v2 gearboxes and wiring solutions like that of the SIG 552. Usually the insulation will be pretty thick, and because it has many thin strands, it might be slightly thicker than cable of the same AWG rating, but with fewer, thicker strands. I would still recommend it for everyone!
Ps. You prolly shouldn't go thinner than 20AWG. Just get 18!