Mercenaries

There really is not a best weapon I stick with carbines assault rifle or shotgun

I, personally, like to use the Prtotype Weapon in the crate labeled "Advanced Weapons Drop."

The overall best combos are this:

Early game:
Carbine
RPG

Carbine
Sniper Rifle (Or Anti Material Rifle as soon as possible)

Later game:
Prototype Rifle
Anti-Material Rifle

Prototype Rifle
AT Rockets, AA Rockets, or for all around RPG

Or mix-up, one of my personal favorite loadouts:
Anti Material Rifle
AT/AA/RPG

For prototype rifle, find 40 Blueprint collectibles, or some South Korean elite soldiers, found near guard towers and specific areas, also hold these.
For Stinger(AA), find 10 blueprints, or kill a soldier with one.
For Anti-Tank (AT), find 15 blueprints, or kill a soldier holding one.

Carbine may be found in a number of places
RPG same as carbine.

Reasoning:
Carbine- very accurate and powerful.

Sniper- Long range death. Kills people before they do it to you.

Anti-Material Rifle- One of the best weapons. It is able to shoot through fences and small walls soldiers hide behind, and best may shoot through the glass of any helicopter, although Sniper can't penetrate tougher ones. So useful when being attacked by a Mi-35, just shoot a bullet through to the driver, his chopper falls down, then you can take the chopper (Assuming it may not destroy on impact).

Anti-Air: One shot kill to almost all helicopters that tracks it down, only the toughest ones will survive, specificaly the North Korean Mi-35.

Anti-Tank- These rockets track like AA, and are a one-shot kill to every single tank, except possibly the Allies'

RPG- Blows up jeeps, damages tanks and APCs, takes down weaker buildings. Very common and all around destruction.

best wep is Street Sweeper from Cheat Weapons Drop =D

My two personal favorite weapons are the prototype rifle and the anti-material sniper rifle.