True, the Corsa sounds awesome. It's also awesomely expensive.
Borla, Flowmaster, Magnaflow are all good systems. It's a matter of what sounds good to you, not what sounds good to us. It's hard to pick a system for someone else.
Having said that, I like the 18" Magnaflow mufflers. Not too bad while cruising, nice deep tone, hell unleashed when you open it up.
Borla is pretty much like the Magnaflows with just a bit higher tone at speed.
Flowmaster has so many different setups now that I haven't stayed up on them. I can say from personal experience that the Flowmasters will sound different as they are a chambered design.
The rule of thumb is the longer the muffler case, the quieter the muffler.