My personal opinion, I would say the AUG would be the first choice. I have a Tokyo Marui AUG SR and it's great. I would imagine that the CA is similar; if so, servicing is easy and you can use a twin type battery that will last all day. The AK variants are also excellent in most regards. As Alexei stated, a full stock AK can store a large battery that you can rely on without problem. Servicing for the AK is very simple as well. Both the AK and the AUG utilize the Version 3 and coincidentally, I recommend both over your other choices.
I wouldn't choose the 416 as it is essentially an AR-15 variant.
As far as the other recommendations, wildcard suggested a FAL, and I corroborate his statement that it would be more original. As far as M14s, if you went that route, I would go with a Tokyo Marui. I find that the Marui M14 out of the box is one of the more accurate and long ranged rifles in airsoft production. When upgraded, they become even more efficient. Finding M14 upgrades is not difficult either. Most major Airsoft retailers keep parts for them. I liked them so much I bought 2, and with the G&P short barreled EBR set up the gun is not much longer than an M4.
Anyways, if you know people with the guns you are interested in, I would try them out and see which you like best.