255/85r16
It's good to keep some threads alive, most related information in one thread instead of several makes it easy to find in the future. I think much of my reply below can be found in other related tire threads:
http://www.expeditionportal.com/forum/showthread.php?t=12879
Bighorns
I'm certainly still a fan of the Maxxis Bighorns, particularly because they are offered in 255/85R16D. Buy them for their looks and/or performance if you like, but be aware they may wear a little on the fast side. It's a nice compound for traction but I think they are soft and not a particularly long wearing tire. For a mud-terrain tread I love them, and my car likes them to. Depending on the on-road conditions you have, you may find other tires with better traction when it's wet/icy, because of more/deeper siping. Not necessarily other M/T tires, but other designs that might be better all around on-road rubber.
Cooper S/T
The Cooper S/T is a lighter tire with less void (but still quite a bit) and a harder tread compound with a reputation for longer wear. The harder compound may cut/chip more easily that the softer Maxxis Bighorn depending on your off-highway surfaces and driving style, but that's part of the tread wear trade off. (However my siped Bighorns are cut more than my S/Ts because of the aftermarket siping and type of use they were subjected to.)
I know the tread is long, but there is lots of information about the Cooper S/T in the specific thread on this tire. I don't think it's the most rugged tire, probably average to slightly below average because it currently only has a 4-ply tread in the 255/85 size. You should also note (see the other threads for pics) that the thread face is very narrow, even for a 255/85, and tapers in noticeably from the sidewall. I still like this tire, still have a set, and was considering giving them a spin for a one-thousand mile winter road trip this past weekend.
KM2
I've run a couple sets of the BFG KM2s, not for tons of miles, but I do like the tread pattern and carcass flex even if my 4Runner does not (alignment challenges). The KM2 also has a very narrow tread surface for a 255/85, though they don’t appear as narrow as the Cooper S/T because they have beefy sidewall lugs that the S/T does not.
Others
None of these tires are the 'toughest' in the 255/85 size, that title probably belongs to the Toyo M/T followed by the Toyo M55, followed my the Maxxis Bravo (an all-season LT tire). But you didn't say you wanted the 'toughest' tire which is a good thing, the toughest tire is not necessarily the best tire. For example the Toyo M/T can be a stiff, poorly flexing tire unless on a heavy vehicle or aired-down very low. They are also expensive.