"Best" is relative.
Best at attack, best at survive, best against ships, best against squadrons, best in mix?
For best in mix (or in general) i would say the YT2400 for Rebels. Just because of the rogue. Followed by the A-Wing, because of the great mix of anti ship, anti squadron and cost.
For imperials it is harder. There is no best in general. They are way to special in all kinds. And if you just ignore the rogue and speed, it would be:
Best for attack against squadrons: TIE Interceptor
Best for attack against ships: TIE Phantom with Sloane, Firespray without Sloane
Best survivable: TIE Bomber
My favorite for best in general would be TIE Defender and Decimator (depending if you need the rogue).