Now I could spend my whole day talking about NES games because truly back then the gaming industry wasn't catering to casual players and wanted everything for the most part to be a challenge. The fact that they did this makes these games still viable today. I don't want to make a NES list only so in this intro I will give a head-nod to the fact that NES had the hardest games I have ever picked up in my entire life (Battletoads, Punch out, TMNT bomb defuse underwater, Ninja Gaiden, etc.) . So I'm just going to make a short list of games that make me red in the face and still would to this day.
The Water Temple in my favorite game Legend of Zelda: Ocarina of Time, just a confusing place with hard battles and plenty of anger to build up as you wander aimlessly through it. I recently played through this game and still spent days in this place refusing to look up any guide and fighting my way in circles.
The underwater Weapon named the Emerald Weapon in Final Fantasy 7. With the timer on and one bad ass boss I hated this battle. Breeding Chocobos to get the Knight of the Round Materia was also a pain. I have not done this since I was an adolescent but I remember how hard and how angry it made me.
The final boss in Half Life took me weeks of playing to actual get the strategy down to beat this guy. Between jumping all around the room and evading his glowing balls this boss battle was a true challenge but worth it because this game is still amazing.
C'Thun was a raid boss in World of Warcraft and by far the hardest boss I fought in the game in my years of playing it. This was a vanilla 40 man raid of the Temple of Ahn'Qiraj and his battle and this was before Naxx or The Burning Crusade for those who are new to the game. It's hard to describe how hard his phases were and just how worthwhile you felt after downing him.