If you literally mean anything but a sniper, I'd reccomend the M14.
If you'd use a burst gun, I recommend the m14 with silencer and a scope, either acog, thermal, or RDS, depending on your play style.
If you want an automatic gun, I'd go with the G36, good damage and low rate of fire. That means you can down an enemy from behind, and they can't turn around and spray you down, while still having good accuracy. I'd run this with RDS or holographic.