It's a very big question, because everything has its pros and cons.
Recon has a longrange ability, and the ability to destroy tanks with your C4. Only a low amount of ammo.
Medic has alot of ammo in their weapons, and the power to heal and revive which gets you alot of points.
Assault has a noobtube with infinite ammo, also to supply to people.
Engeneer has RPG's to blow up everything, and repair for alot of vehicle healing :P
I'd have to choose for Assault. I hate running out of ammo