Your engine options are going to also depend on the kits you want to power. If you are looking at BVM kits, then your best bet is probably the BVM line of engines (.91 and .96), although some have run the OS .91 in the BVM fans with success.
But if you are looking at kits for the 5" (Dynamax) or 6" (Byron / IronBay) fans, then the OS.91 is your best choice.
If you are looking at the smaller sized kits for .45 sized power, then you're best bet is the OS.46. (Or K&B.48 if you can find one)
Honorable mention goes to the K&B line of ducted fan engines, although they are harder to find nowadays. They made the old 7.5 (.45), .48 and the .82.