I think we'd need to define efficiency.
But assuming it's just speed
26x26 in theory ought to produce the highest speeds, but in practice...all of the really fast bikes seem to be faired, and most of them use at least one 20" wheel.
20x20 ought not to be the most efficient, but darned if I can tell any real difference between a Rocket and a VRex other than looks. Once you're on it, you can't see the back wheel.
The gearing of smaller wheels does take some thought. But if you can produce power over a reasonable RPM range, it's a non-issue.
26x20 is a decent compromise, but IMO, a lot of the supposed "really big differences" simply don't exist as other than placebo effect for anyone who rides less than 4k miles a year or so. If you ride more than that, you probably have a pretty good idea of what you want anyway.
Not real helpful, was it?
Just get the bike you think looks best & is most comfortable. You'll ride it more, and as a result you'll be faster than you'd otherwise be.