My vote definitely goes to the Helmasaur.
As soon as you walk through the room, you can tell they intended to give him a menacing appearance. A size which staggering at first glance compared to Link, and physical characteristics such as a spiked tail and long claws that certainly do not make him any less intimidating. As you begin the battle, you're given the impression as though he is a slow-moving, predictable boss. Clearly not as threatening as he appears to be. However, as soon as you break through the mask, his behavior changes. He speeds up, he's reacting more violently than he was previously. It is surprising and the boss surely isn't a let down in design or behavior; overall a fun battle with a good quality boss.