You have a good selection to chose from. The first point is cost, there are options ranging from a few hundred to a few thousand, and generally, performance goes up as you spend, it's not just a name, but there are a few exceptions!
Ebay has a few systems regularly listed, outlaws, slip ons etc, these will be mostly sound but will give a little more pep to the bike.
Then you could do an ebay cross over pipe and TORs (Triumph off road) mufflers. The TORs are without cat, larger internal diameter, sound good but not particularly loud. Very livable. Intake and TORs with the cross over can land you in the 140s for whp SAE.
Then there's the Jardine full system, sounds great, is loud, does give more performance hard to find, its out of production.
Then there's paul, viking exhaust, who makes a beautiful system, I haven't heard it, and as far as I know there are no posted dynos, but there's no reason to expect it doesnt sound good or perform well.
Then there's the in production performance oriented full systems which have quite different sounds but all add a good bit of power, DESPITE what anyone says, this community STILL hasn't seen a back to back, same bike same dyno same day comparison of the three to see which makes the most power on a R3 without a modified motor, in order of price they are:
Carpenter Racing
CES
REBAND