If weight and power isn't a major issue, get a OS, TT or Magnum plain bearing .15.
If you want more power, the Norvel is a great little performer but the muffler on the .15 sucks- a heavy clip-on type.
I've got an AP yellow jacket .15. It's powerful and light with schnuerle porting, BB's and rotateable rear exhaust.
At a little over 50 bucks, it's the best value of them all.
Hope this helps!!! Kirk