The hardest bosses I have faced?
Psycho Mantis: Metal Gear Solid 1 PS1. He controls you, reads your memory card, makes your controller vibrate, then disconnects your console from the tv momentarily in battle. When you aim at him, you look through Mantis's eyes. He can fly, be invisible, and makes Snake break the fourth wall (where the character in a movie/video game interacts with the viewer)
Feral Chaos: Final Fantasy Dissidia Duodecim PSP. Feral Chaos is level 120 with over 130,000 health. The most you can have at level 100 max character is around 11,000. After 90 seconds, his equips increase his attack and defense by 450%, making it impossible to damage him with bravery attacks and he will usually instant break you with his high attack. He is fought first when you are level one, which makes it the hardest boss battle in Final Fantasy. While it is possible to defeat him with level 1 Lightning, upon beating him, the game acts as if you lost.
Through the Fire and Flames: Guitar Hero III, Legends of Rock PS2 or Xbox360. It is without a doubt the hardest song in the entire series. It contains one of the highest NPS (notes per second) in the series with 16 nps (beaten by Black Magic Women with 17 nps). Its rapid hammer-ons and pull-offs combined with rapid strumming and tricky solos make it the hardest song and only the best can defeat it. Currently, the highest score on it is over 1,000,000 points. While being able to just complete the song is an achievement in itself, the hardest challenge with it is 100% accuracy on expert. Only a handful of people who attempt 100% actually suceed because 75% of all videos on Youtube claiming they completed it 100% are actually bots that hit the notes for you. The ultimate challenge with TtFaF is using another instrument to play it, and this was achieved by using drums...PURE INSANITY!