I like my model 21, but for sheer brute toughness, nothing comes close to my Ruger Super Redhawk in 44 mag. It's about the only regular production gun I've ever seen that can kill just about anything in the lower 48 you can put in front of it out to 100 yards, and still be used to hammer tent pegs into the ground (I would never even think about doing that with my S&W 629, let alone a stripped down version like the mountain guns).

Even with great stats like that Sig, I would take a good quality revolver over any semi-auto for toughness. Still, in a bug out situation, I'd probably grab my Glock before my SRH for other reasons, go figure.