For me it's the M4 but everyone has chosen it already so I cant repeat the obvious...
AK-74u
Cheap, hard to notice on your back, high fire rate and reasonable recoil. It uses the popular 5.56x39 round which is a blast to shoot ingame and IRL. It isnt the most silent gun out there but hey, we've seen louder guns like the AK-101. Attatch a compensator, red dot sight and a deafult stock and you have yourself a killing machine that, for 11k price, is more than worth it.