So as we all know, the US has been using the AR-15 and it's derivatives for literally decades now. I personally think the rifle has run its course and needs replacing. The reliablity of these rifles is nowhere near that of modern rifles (like the FN SCAR). Heck, even the old AK-47 is more reliable than the M16. So what do you think is a good rifle to replace it with? I think the SCAR is an excellent option myself.
The AK-47 is just downright easy to use. It requires very little maintenance and can last a long time without it. That's why you see the terrorists use it so often. It's a very effective weapon. The SCAR isn't a bad weapon, I'd go with it too.
The US should develop a gas operated, reliable weapon, similar to the AK-47, for use as standard issue. If they deem that the AK gas model cannot be improved upon, then I have to agree with the SCAR sentiment, it is a great weapon.
Anything that can put a 7.62 NATO round down range . M14 with a modern sight system should do what it says on the tin.