Everyone plays a little differently and as you gain experience you may change how you play. These are what my opinions are of the unlockable weapons. In no particular order.
SNIPER = M95 - Not much of a sniper type player, so I haven't unlocked it. Way under powered for use against soldiers (same damage as the M24). It's nice that you can take pilots out through the cockpit windows.
Assualt = G3 - I really like playing as Assault, but losing the gernade launcher & smaller mag capacity makes this a big negative for me. Has pretty good damage and works better in semi-auto mode at the med/long ranges.
Special Ops = G36C - Nice unlock. But, not enough of an upgrade over the M4 to warrant unlocking for me, yet... You loose the red dot sights, but you get used to the iron sights.
What I have unlocked thus far:Medic = L85A1 - I like it as an unlock. Works very good as a counter sniper weapon or taking out soldiers at intermediate/long ranges. Works best in semi-auto mode, get lousy accuracy in full auto mode. Short burst works better than unloading a full mag at once at close ranges.
Supply = PKM - Pretty powerful, but I'm disappointed in the accuracy in ALL the belt feds when shooting in long bursts. Short burst are much more effective, when trying to pick soldiers off. I like using it against Helos & unarmored vehicles.
Anti-tank = DOA-12 - Awesome at close ranges. You can easily mow down 2 (maybe 3) players at once in close combat with some practice and proper aiming. But, you have to watch out for the long reload times.
Engineer = Mk3A1 Jackhammer - Powerful full-auto shotgun. Great for close combat. But, you run out of ammo very very fast, before you have to reload. You better pray that you don't run into more than one guy at a time.
WEAPON UNLOCK Info: ubar.bf2s.com/unlocks.php