For me the things that make for a good engine, one that might interest me, is firstly light weight. I'm baffled why you can have one .60 weighing ~12 ounces and another weighing close to 20. I prefer rear exhaust, it's neater, gives cleaner lines to the model, easy to use either a pipe or muffler. A carb that's of the correct size for the revs normally flown. High compression to take advantage of low or zero nitro. Timed for torque and not HP. Removeable (unhardened) stud for the crankshaft to save the crank from bending in the inevitable crash. Transfer hole(s) in the piston to help it's cooling and lubrication.

I can't think of any engine that meets all of this but some come close. The OS VF's come to mind (they're too heavy though) and one or two others.