I honestly enjoy using most of the weapons in the game other than sniper rifles. My top 5 as far as kill count, in order, are:
1. MSMC- usually just run with Long Barrel
2. MP7- Long Barrel
3. PDW- Silencer (stealth class and PDW is great when facing multiple targets due to mag size)
4. FAL- Stock and Reflex
5. AN-94 - Stock and FMJ