I don't believe that there is an ulitimate engine and pipe combo as any top line set-up will perform well for a particular purpose... Some motors are good for long flowing tracks and others are good for tight technical tracks. Then there is the motors that are somewhere in the middle... It's a trade off really, do you want top speed or low down punch?
But if i had to pick 1 motor? I'd say the OS Speed... Why? Cos i own 1 and love it!