For me;
Assault: AN-94, G3A3, and L85A2 (unlocked through B2K assignment), with Reflex RDS, Bipod and Heavy Barrel.
Engineer: HK53 also known as G53, with Reflex RDS, Foregrip and Laser Sight.
Support: MG36, M240B, RPK-74M, M27-IAR, Type 88, and PKP, with either Reflex RDS, Holographic RDS or M145 Sight, ALWAYS Bipod and Extended Magazine.
Support: M40A5, even though statistically it's the worst sniper rifle, I love it. 8x Rifle Scope (no gay ass 12x), Straight Pull Bot and Laser Sight.
Some people say the majority of the weapons and attachments I use are terrible, yet I just see it as a way of improving your skill. If you do exceptionally well with shit weapons and attachments, you'll rape guys who use "OP" or overused guns. Just my 2 cents. :p