A Polychain should be very efficient. I seem to remember reading that the polymide flat belts are about the most efficent you can get. Siegling and Habasit both make them.
I am guessing the Polychain is around 96%. Why do you need even better?
Would the sprocket diameters make much difference? Intuitively, the larger the diameter, the less tension differential the belt would require, BUT at the same time the higher linear velocity?
I think it would be splitting hairs, but just curious.
Helical gears with helix angle just high enough for 1.0 or greater axial contact ratio, or spur gears would be about 99% efficient. Gears have much tighter alignment requirements than timing belts.