No. 2 - Grim Tales
I’ll never forget that when I first got OSRS membership, Grim Tales was one of the first quests I (accidentally) started.
At the time, I didn’t have the skill requirements to complete the quest, of course. But I’m pretty sure that if I did, I could have knocked out a Master quest within a few minutes of joining the ranks of the pay-to-play world.
There is really nothing remotely challenging about this quest. The boss fight against Glod is not a Master-level fight, even when compared to some of the other relatively straightforward boss fights in Master quests that arguably should not be classified as Master quests.
The entire quest is pretty much a spin on real-life fairy tales, and while it does have a prerequisite in Witch’s House, it is effectively a standalone quest. So from a pure OSRS perspective, I can’t really give this one the benefit of the doubt when it comes to a strong storyline elevating it from Experienced to Master.
With all things considered, there are probably a couple of Intermediate quests that would rank higher than this one in terms of difficulty, and those quests have storylines more integrated with RuneScape lore as opposed to outside stories. Sure, those references keep things entertaining, but not to the extent of justifying it as a Master quest.