Imagination, because imagination can turn a squirt gun into a portable rail gun.
Best weapon is the shitty lag comp - irrelevant to what gun you are using if you are 1-2 seconds ahead and can shoot them before they see you. Well I go 50 plus kills everytime. But this happens very little as having super fast fibre means I'm always on the bad end. Funny how I use my 3G connection thru my iphone and I dominate games with ping rates that are embarrassing.
No such thing as a best weapon. Just use them all and see what you like the best/do the best with or use whatever you enjoy using.
My best and favorite weapon is the crossbow, but its far from the best weapon in the game.
I'm finding the Chicom CQB with select fire,long barrel, laser sight to be a fun gun to use, good accuracy and rate of fire great for hip shooting and ads.
If you have not tried it and can handle the recoil the Skorpion with rapid fire is a beast and sounds evil when it shoots
Personal fave secondary either crossbow triple shot or RPG ( great for one hit killing stealth choppers/escort drones) I find these help if the enemy spawn camps especially in Nuketown
I like the LMG's because they actually kill people - My favorite being the MK48 Silenced - get tons of double and triple kills. Don't forget to throw on Lightweight for LMG run and gun goodness.
The Tac-45 is the best gun in the game in my opinion it provides a consistent 2-shot kill radius in core and 1- shots in HC if you can play well with a pistol you can out match any gun though this applies to all guns but pistols are very powerful in this game so i take advantage of it just yesterday i got a 10 man feed in HC domination just 1-shoting guys.
I get the best results with the an94, but it doesnt take long for the lobby to follow suit and equip it themselves!? So to avoid tougher lobbies I role with a silenced fal, because people dont rage and call me out on my noobnees with a 94.