HK416s are nice. They aren't too common (yet) but are making their way into the common list. Upgrading them isn't a hassle like some of the more uncommon-but-not-common rifles out there.
AUGs are quite uncommon. I only know of a few people who have one, and actually use it opposed to having a wallhanger. If you can get one properly kitted and built well, it's a beautiful rifle. Not to mention the low-cap magazines with fake rounds are already in the country, and not for insane prices.
Just a note, I'd avoid the SiG552, or just the SiG (rifle) series all together. There are some affordable upgrades for it, such as a metal handguard set, but there's little to no replacement parts readily available. It's one of those guns you practically need to get two of so you have one to donate small parts like screws and pins in case you break/lose a set. I find it's quite rare to find someone who actually pursues the entire SiG line (including pistols)
M14s are also kind of rare. They're nice, but have a hard time getting into the country I believe (could be wrong, don't quote me on that). They're also really quiet, or at least the few I've heard fire are quite quiet compared to say an M4. Kind of odd, not sure what causes such a sound buffer.
A -good- AK is hard to find. AKs in general are a dime a dozen, but few times have I seen people who pursue an AK, build it durable, playable, and realistic. They're not very expensive to upgrade from my understanding either. Metal bodies are what, like 80 bucks a pop? Most metal bodies are $200 or more. Only situation sometimes is getting an appropriate vest for the mags, which even then, with about 10 extra minutes of your time, you can find a suitable vest and pouches.
Primary: Custom WE M16A3
Secondary: KWA MP9
Side: Custom WE M1911A1