Depends what you want.... loudness, tone, look etc

The only ones I have experience with are Milltek and Piper

Milltek is raspy and the tailpipe has a standard look, with the resonator they are supposed to be not much louder than standard, non-res is quite loud but not unpleasant inside the car.

I had a Piper for a while, gave a nice deep sound but was very droney inside the car. Loads of tailpipe options and seem to be made of better material than the milltek.