The M16A3 is a default weapon but has a bad fire rate and a lot of recoil. It takes about 1 whole clip to kill someone depending on your HP. Scar is a lot better than any of the other guns. Which would be the Cobra and the M4A1. Cobra is better than M16A and M4A1.
Other decent guns:
G36
UMP
Uzi
AK47
AK74
MP5
AUG
Hope this helped you! I am currently using the MP5 without silencer and it takes 2 shots to the head for a headshot. To the chest about 5-6 bullets. Best gun in the world = MP5!!!
Here's a little correction, actually, somewhat big.
M16A1 has little to no recoil, I believe you're getting the gun mixed up, seeing as it's the default, it's obviously going to be quite easy to use.
It's actually 5 shots in the chest for a kill, one shot to the head. By the way, ALL guns are 1 shot to the head, well, if not, most. Seeing as I've shot once at a head with the default pistol, which instantly gave me a headshot. (Basing it off that, correct me if I'm mistaken.
Seeing as I pick them off quite easy, usually using one shot to the head, right after they spawn. (Yep, I'm a spawn killing nub.)
Best gun in the world? No.
Decent gun, yes.