FAMAS, AUG, or AK-74U
Oh look, I became good in Black Ops! Never mind the fact that the snipers are terrible, shotties aren't that good, etc. And BLOPS has some shitty ass hit detection for sure.
Ideally, I'd like for them to return to their MW roots in MW3. Hell, I'd like for them to get rid of the variable killstreaks and return to the 3-5-7 set up in MW. A helluva lot less camping for killstreaks in 4 than in MW2. The gun balance was pretty good in MW as well. The only outclassed weapons in CoD4 were the SAW, MP44 and the R700 (albeit the camo on that gun was so sexy). One could argue that the UZI was outclassed as well. Every other gun had it's own advantage and could be reliably used. Yes, the M16 was a very good gun, but it wasn't game breaking like the MP40 in WaW.