Definitely the four-phased final boss of Twilight Princess. If I had to pick just one phase, though, it'd be either Dark Beast Ganon or Dark Lord Ganondorf. I love the former because of how effectively it works as the climax of TP's "animal/sacred beast" theme. Of course, Ganondorf's Twilight form would be a giant, "wild" boar! The latter, while not being all that challenging, is just so powerful within the context of the game.