Though not the hardest boss, when I think "harrowing" the first thing that came to mind was the 11th colossus in Shadow of the Colossus.
Kind of ironic because it was easily the smallest colossus in the game, but I remember I was having a lot of trouble beating him (also I was a lot younger and not as good at video games) and so I had logged about an hour or so in the fight and was starting to get the hang of it and was hiding out in the lake to heal and not die and it was just so tense and epic and when I won I was so relieved.
Favorite game of all time, still.
The bull-like one that you had to scare with fire initially
Kind of ironic because it was easily the smallest colossus in the game, but I remember I was having a lot of trouble beating him (also I was a lot younger and not as good at video games) and so I had logged about an hour or so in the fight and was starting to get the hang of it and was hiding out in the lake to heal and not die and it was just so tense and epic and when I won I was so relieved.
Favorite game of all time, still.