There is no best. SBP is the most versatile and can be recommended by anybody who has used it. Not many members have gotten the arrow exhaust but I'm sure it's good too, but less customize able. The CNS carb sucks. thatsdax's RT carb is good, I like the semi auto choke. I heard the speed carb is good too. All require tuning more than likely.
SBP makes the only shift kit that works right. I don't think thatsdax makes his anymore.
You might consider getting thatsdax's GP50 clone motor. You'll get a **** of a lot of more power than these motors can ever make (without spending the equal or more amount of money). It will require more work to mount it though. You wouldn't have to bother with some electric hub if you were trying to go without a pedal start either. I personally don't think it's a good idea to make a hybrid bike as a first build. You mind as well buy the right engine for the job than mix and match things that will end up costing you a lot more.