Ocarina of time, just because they were so varied, and the first time you face them, so intimidating

The first game I ever beat as a kid was Super Mario World. While Bowser might not be an impressive boos as a kid I was pretty damn intimidated. I mean, he was HUGE, was on top of a castle, the music was intense and he delivered attacks nonstop.

Then I played Starfox 64 and was really impressed by Andross. I just loved how when you face him his face really covers the entire screen. When you fight his brain...I was floored.

I know Final Fantasy VIII is hated by many but I was really floored by Ultimecia. The music was pulse pounding and the fact that you could lose your party member permanently made it very intense.

Ocarina of Time. I know. Boring answer.