This subject has been pretty hashed, but I'll put in my $0.02 anyhow.
For bottomless mud like you intend on wheeling in, the best tire, imo, would be a 35" that is as wide as it is tall. That would basically allow you to "float" across the top of the mud almost as if it was pavement. A wide tire exert less pounds per square inch than a narrow tire does. Wide tire = less psi = less force moving the mud.
But for most everything else, skinny is better. In rocks, a narrow tire with an aggressive lug will exert a lot of force on the rock to pull it up and over the obstacle. On the street, a skinny tire (to a point) performs better than a wider tire, and with better fuel economy, due to less rolling resistance.
A vehicle is designed around a certain size tire and contact patch, which affects the psi exerted on the ground. When sizing larger tires for a vehicle, the size of that contact patch should be kept in mind. It is best, imo, to keep near the factory contact patch. Unless you're building for mud or sand, then go as wide as possible.