Smough and Ornstein has to be up there (theres a lot to pick I'll just yap about this one for now). Running through the castle in Anor Londo and seeing the Drake heads that adorn the walls only makes you wonder what kind of fight lies ahead of you. You get to the outer quarters of the cathedral and it's guarded by many Royal Sentinels, large towering knights with Halberds, preparing you for the Behemoth of a fight this would be. I personally play without summons and do all my boss fights 1 on 1 so this fight took me a really long while as a kid. Walking through the mist and immediately you get hit with such an intense and terror inducing OST while a giant Burlap Sack of Potatoes charges at you with his hammer in the floor to throw you 15 feet into the air, and his friend who whizzes at you in a blink of an eye. So you do what one would normally do and kill one first before the other. Easy enough, we have done this multiple times at this point in the game. But then the surviving one CONSUMES THE ENERGY OF THE ONE YOU JUST KILLED. So now you have a Bigger Version of Ornstein that does a butt slam with an AOE lightning effect (same that Smough would do) OR a GIGANTIC Smough that just has lightning on his hammer... lmao. One of the most difficult and most rewarding feeling after beating for me personally.