I'll go out on a limb and say Royce Gracie in his prime may be the second best MMA fighter simply because he really stepped up to answer the question of which martial art is the "best?" He stepped in the octagon with opponents of every weightclass and discipline, and dispatched them. Arguably, if Royce Gracie had not been a key factor in the UFC's initial success, BJJ and submission defense may not have been a major factor in a fighter's repertoire until later. Royce showed that despite a fighter's weight advantage and knockout power, they better improve their ground game and either have titanium joints or be able to fend off a submission.
My favorite fighters of all time: