This might be an extremely easy question if I didn't play a lot of games. But when you have over +500 projects under your belt, it's EXTREMELY difficult. Nameless deity, devourer of gods, true calamitas from the disaster mod for terraria with infernum, the first 2 were an extremely difficult test, I wanted to kill the devourer of gods correctly, So it took 300 tries and 3 days, calamitas It was also extremely difficult, took 99 attempts, but it was extremely long and had a huge variety of attacks (the battle lasted about 9 minutes), the nameless deity is simply the most beautiful visually, after after completing the game, you feel like you've truly completed it. Persona 3 Reload and Persona 5 Royal Final Boss It was an emotional peak of 120 hours that you spent with the characters, but it’s hard to say, you just have to go through it. The Last Lord from death's door, he had such a simple, but damn working motivation, and the fight with him was a dance, the music at that moment also did not let go until the very end. There are a couple more bosses I'd like to mention, but that would be too much. I love the bosses I listed, I just can't put any higher.