what are the best weapons and powerups?

Scar, and that depends on your playstyle.

plently of posts on this topic dude already and some have gone into great detail, including myself.

scarab

Depends on map, game type and style.

Plain and Simple the best Weapon is the SCAR. Crazy good stats for both close to semi long range fighting. However you can still play viably with other weapons your just going to need to change your play style around and realize you can not win most head on 1v1s.(though if you know how that will not be a problem.)

I use the Scarab as i had slightly better accuracy over the scar & feline. My set up is Scarab with a silencer & extended mags, Stealth enhance++, Energy Transfer++ & Mobility Enchance++. Epic class & i usually get around 20 kills most games with a few deaths. You can pretty much get around the whole map super fast & in stealth then you have the advantage of going straight back into stealth when you get a kill. Watch out for me & my crew SPC as we will kill you & also annoy you in game chat! XD

is SPC a clan?

Feline is best at any range, the fact that it has an Assault Scope as well reinforces the points. I had people sniping with a Feline 30-50 metres today on ShipYard. It's f'ing ridiculous for an SMG. On a 1-on-1 it's hard to lose to anyone with this weapon.

Scarab is second best, the sound is also amazing with no suppressor and looks awesome.

I'd say the Scar.