27.5..........or for that matter 26 will have a shorter turning radius and will be better for turning in tight spaces.

29ers are better for rolling over large objects.

I tried a 29er a few years ago and did not like it. It was ungainly. I prefer my old 26er Cannondales with the Headshok.

None of these are any better climbing hills than the others unless you factor in that a smaller diameter tire will give you slightly lower low gearing. That is really a moot point since most mountain bike gearing is already very, very low.