Hmmm. I'd have to say "all else being equal" the sloop is the most efficient overall - as far as powering the boat. They cost more, are more complicated than a cat, but are well understood and easy to tune. When was the last time you saw a catboat in the America's Cup?
Least imaginitive and boring to look at, but it goes to windward better than any other rig. Crack it off the wind a bit and it becomes less so.
But look at that big head-cracker boom you need for most cats (the sub 20 footers not withstanding). I had a Gloucster 11 sloop - about as small as a sloop goes - and it was a great little sailing boat.
About the best sailing cat boat I can think of is a c-scow. They don't point all that hot, but they tack almost instantly and there is no delay in running the jib in so they can hold their own in upwind duel.