depends what you mean by best? best performing? best value for money? best performing would probably be the mhr kit, but imo value for money would be an iron sport kit, either malossi or polini and u can get them at http://www.scooter-center.com
Get malossi mhr rep, jet it correct, use fully synth oil and dont abuse it to much e.g rev bike on stand 4 ages n shit it should last long time