The two rear tires that work the best without a doubt are :
1) Maxxis iRazr rears. These have some totally insane hookup and grab as well as sliding for going in and out of the trees. These work awesome, only downside is they wear quick and are heavy. These are the ONLY tires for a mud race, although I haven't tried tire #2 in mud and they're probably good as well.
2) Kenda Knarly GNCC version. The GNCC Knarlys were partly designed by Jeff Stoess ( a woods racer) and these things hook up just about as good as the iRazrs everwhere. They are awesome tires as well, IMO. If you don't have the cash to replace the razrs every 6 months (trailrider) or every 4 or so races, then get these. These are also lighter than the iRazrs I believe.
You can not go wrong with either of those. On a side note, the iRazr fronts absolutely suck!!! Do NOT run them, or you'll end up in a tree, IMO. The Kenda Klaw fronts are totally awesome. They corner well, don't collapse in the trees and have awesome bite. I run these now and soon I'll be running some Knarly rears (GNCC of course).
Note: The std. Knarly XC doesn't do well in the woods compared to the "GNCC" Knarly, so be sure to check that.