I ride mostly intermediate terrain. Depending on the weather, that can mean anything from lots of mud to dusty hardpack. I've found the Michelin S12 to be the best choice for me for a rear tire. It grips very well in everything from mud and sand to almost pure rocks. And it wears very slowly compared to the rest that I've tried. I like the 140x18 (same as a 120 for most other brands) for my 400EXC but the 130x18 (same as 110) worked best on my 250.
The front is much more subjective. It has a lot to do with how you ride, suspension setup, etc, IMO. I really like the S12, Dunlop 752 and Dunlop 756 up front, in that order. They all seem to grip about the same in intermediate terrain with the 752 maybe having a bit of an edge in the mud. The S12 can get skatey for me in the hard pack but it's not severe. It wears the best of the 3 on my bikes. The 756 is the weakest in the mud. All 3 are good, though. I have at least a new one of each in my shed and I'm always torn about which to run next.
I run Bridgestone M-78's (Int-Hard) and M-70's (Soft-Int). I just saw a new tire that Bridgestone came out with that is supposed to be similar but better than the M-78 (that RC and others have been using a lot outdoors)... so I might give that one a try next time around.
IMO - Bridgestones hold up a little better and longer than Dunlops and seem to hook up better for me (wash out less). Their sidewalls seem a little stiffer - helping for better tracking through corners. Plus I have been a big Bridgestone user for years now!
The IRC M5B is a great rear tire, as I couldn't ever get this tire to break loose. The knobs are like skyscrapers, though. Another good tire is the Dunlop. In front, I have used 752s, and they are pretty good, but I will try something else next time.
S12 rear. The front tires I have tried, from best to worst are, dunlop 756, pirelli mt44, michelin s12, dunlop 752, dunlop 755, michelin enduro comp III. I ride in mud, gravel, hardpack, rocks, sand in that order (very little sand). I'm tempted to try a 756 rear, even though most people who recommend them also recommend the 755f...