Some of the things said about skinny tyres dont make sence to me. If they are skinny they will sink in mud rather than spread the load, that will make the riding hard work. Less rubber also means less to grip slippery roots, rocks, etc with. If the nobby bits are spaced far apart then they wont clog up, but that applies to any size tyre.
I find my trail rakers only really work on muddy wet grass, as soon as its rocky or rooty they slip, if its not that muddy they drag. When they work well is very muddy slop its hard work because of the mud irrespective of the tyres.
I find good all round tyres that are not worn out will work fine in most conditions. Since most of my riding is in a variety of conditons an average tyre will do.