it depends what you want.
Boss if you want to go cheap and have ok performance (they do have potential to rip w/ everything dialed in),
Proma Circuit if you want the best pipe ever. Best because it offers awesome low/mid-range and makes stop and go riding really fun. On my za50 its like the coolest thing ever, but not fast.
I don't really like estorils, but they probably are the best alternative. Huge header, for good low end, and if you pull the restrictor out of the rear cone, you also get really nice top-end. So yea, its the most work but probably offers the best all around performance. Too bad they are butt ugly, stick out really far look tacky.