Solid Snake may be considered the main protagonist of the Metal Gear Solid series, but let's face it: he wouldn't even exist without Big Boss.
Boss' first appearance was in the original Metal Gear, though he didn't become a playable character until much later when Metal Gear Solid 3: Snake Eater was released. An unfortunate encounter with his former mentor leaves him with countless bruises, dislocated joints, and broken bones; later on, he even gets his eye shot out.
Despite all that, he manages to complete his mission, earning him the legendary title -- Big Boss. The rest, as they say, is history.