Shaft wins the vote so far.Why exactly is it that a shaft drive has a greater power loss than a chain?
Have you done the math, or are you just going with "popular wisdom"?
Enclosed chain would be ideal.
Shaft drive is heavy (and much of that weight is unsprung) and inefficient. (right angle power transfers have significant losses)
Belt drive is better, and if drive ratio changes were easy/cheap then I'd consider it. Biggest down side IMO is that failures tend to be sudden, no warning, unlike a chain that you can see/feel going south.
Chain, in the real world at least, is best for me. Cheap, easy to fix/replace, easy to change ratios, efficient and light. Enclosed would cut down on the negative, it would last longer, take less maintenence and make less of a mess.