My favorite gun always depends on the situation that calls for it, i.e. is it advantageous to rush, camp, hide+wait, rambo, confuse enemies with the sound of their own guns, clear a long-distance MG position, remove a hidden sniper, you get my drift...
All that said though, my all-purpose default would have to be the Thompson, and I tend to shy away from the M1 and 30cal since I'm still horrible at them after all these years.