In 1959 it was the Johnson .32, just simply tremendous power compared to the McKoy .35 redhead which was the standard in my C/L crowd.
Now its the Moki/RCS 215cc gas radial. Bought it 2 1/2 years ago, the kit's still not finished, but I run the engine just for fun! I like engines!
For everyday, I have a Saito .56 that just runs great, with a perry pump.