For a stock bike I really like the Toomey B1 Kit. It comes with a good exhaust system, filter, and jet kit. However, the best one depends on your riding style and terrain, along with your future plans for building the engine. Alot of people like different pipes- research and pick on that suites your needs and budget. I would use the search tab on the forum to look for threads about pipes or exhaust and read through the responses.
like blasterboy said if you put a bb on it or port it go with a power pros, dyno port, or lrd pipe. if not anything like toomey, pro circuit, fmf, or shearer is cool.