With Availability of weapons considered
Okay I'll play, but assuming the world has gone to hell then ammo and parts availability have to be put into consideration:
Pistol : FNP-45
Submachinegun : HK or MKE MP5K
Assault/Battle rifle : Gas Piston AR (full auto); UDMC PVAR
Heavy machine gun : M240
Sniper rifle : Accuracy International AW in .338 Lapua Magnum
Shotgun : Mossberg 500
Special Purpose : Rail-attachable 40mm Underbarrel Grenade Launcher (for AR)
I chose the following because they are all man-portable. And although I'd prefer the MG be mounted on a vehicle, if I ran out of gas or my ride got busted, me or someone else can take it off and go on foot.