Rifle: AR-15/M16 variant. It's battle-tested, accurate, and suitable for nearly all modern warfare. M4s are handy too, but having a few in that "family" are good. Ergonomics are good, the rifle's lightweight, and nearly everyone can handle it well.
Pistol: Glock 19. Able to fit most people's hands (the XD-45 can be too big for some), 9mm recoil is easily managed by most people, few moving parts mean less to break, and spare parts are cheap and plentiful. Easy to train on.
DM Rifle: M14/M1A. Accurate, powerful.
SMG: Uzi. Simple, fires 9mm (cross-compatible with the G19), manageable recoil, few moving parts mean less to break, parts are cheap and plentiful. Reliable, durable, and easy to learn.
Shotgun: Mossberg 500 or variant (590?). The tang-mounted safety means that it's easy to train for both lefties and righties. Lightweight, durable, reliable, and mechanically simple.
Sniper: Some variant of the Remington 700, threaded for muzzle-mounted suppressor. Sure, suppressing .308 isn't all that easy, but reducing the sound of the shot will greatly reduce the ability of the receiving party to determine the location of the sniper, increasing the sniper's survivability.