Battlefield Play4Free

What are the Best Weapons? at all Classes?

The most expensive ones to buy with RL cash of course.

Not neccesairly. I'd avise you to try each of them with credits and see which one suits you best. It's often a matter of presonal preference.
Arguably, the most efficient guns are
MP5 and UMP for Engineer
MG36 and SAW for Medic
AEK and SCAR for Assault
M95 and M24 for Recon
XM8 series are also a viable option and I've seen many people running with AUG. Still I encourage you to try the guns for yourself and pick the ones, that you find most effective and / or fun to use.

USAS for Infantry maps. All classes. Make people mad.

Usas+M911