No. 7 - Legends’ Quest
Jagex wanted the 50th RuneScape quest released to be special, and Legends’ Quest certainly fits the bill. The adventure, largely based in the Kharazi Jungle on Karamja, was the longest in the game when it was released, and Nezikchened became the highest level quest boss when that happened.
We have it a little lower on this list than one might expect, simply because you have to fight the same boss three times (in three unique locations, at least) before gaining access to the Legends’ Guild. Even some of the other older quests in the game require variety and the ability to adapt to different strategies.
If this quest were to have been released post-2013 rather than 2003, it probably would not be a Master quest. But it was officially the game’s first Master quest.