Before buying a pipe consider where you will ride, how you ride, and at what revs you need power.
There are quite a few good pipes on the market but some of them are built for specific needs. It is a matter of choice, power low down, power midway, power at the top end.
Some pipes will spread the power over a large rev range. You need to check out the makers specifications and buy accordingly.
My choice for MX (because thats what I ride) is DMC Alien, not cosmetically the best looking pipe, or the easiest fitup, but it works like a charm.