For gas, my favorite is the Rotax 503, no better gasser ever made. I liked mine so much I bought another and now I have 2.

For glow, I'd say my old OS .61 fsr was my favorite. I still have pieces of the crankcase in a box here in my apt. If it weren't for the too-small bearings, my 1.40rx would have been my next favorite because it ran so well during its very short life. I'd also like to claim my 1.60fx, but it's still NIB awaiting an airplane so no running data on it yet....

LS