IMO the simex extreme treckers are the best mud tires onthe market.
some others have the same profile like the silverstoe MT-117 extreme.
whe i air down to 8 psi my cooper STT are nearly as good as my buddies simex.
the BFG MT or wrangler MTR are not as good in mud.
another good mud tire is the baja claw:
or better the maxxis mudzilla: