No. 6 - Mourning's End Part II
Finding a spot for Mourning's End Part II was inevitable, but finding the right spot was a challenge because of the unique challenge this quest presents.
There is no true combat in this quest, yet you are attacked throughout the Temple of Light by Shadows, and their damage can pretty easily stack up. Yet even ignoring those attacks, the lack of combat isn’t enough to keep this one lower than No. 6 on our list.
Getting through the light puzzle, with all the mirrors and crystals in the right places at the right times, in the Temple of Light was largely considered the single hardest non-combat task in the game when the quest was released in 2005.
As many players will tell you, thank goodness there is only one true obstacle to fail, and thank goodness Jagex opted against making the puzzle solution unique for each individual. That said, props to you if you can make it through without a guide. Even in modern-day OSRS, doing that probably makes this one of the top 10 most difficult quests.