After playing through the first level of God of War, where you have to climb up to the top of the ship just to get to the middle head of the Hydra, I got to thinking "these are the kinds of bosses I like best". The bosses who are so massive in scale that you might just wet yourself wondering how the tiny creature you've been playing could possibly win the battle.
A few others come to mind: remember the Sta-Puft Marshmallow Man in the Genesis version of Ghostbusters? Though you don't see the whole character, he's clearly way bigger than you are. Likewise for the end boss, Loki, on Ghouls & Ghosts and the fifth level boss in Forgotten Worlds, all Genesis games.
Think about it. What bosses are so incredibly huge in scale that they made you think... "that's one really BIG boss right there"?