No. 5 - King’s Ransom
Unlike the previous four quests listed here, King’s Ransom is actually classified as an Experienced quest, rather than a Master quest. However, I don’t think it’s a stretch to suggest that it should be classified as at least one tier lower.
In and of itself, King’s Ransom is simply not an Experienced level quest. It is one of only two (arguably three, depending on your point of view) Experienced quests which includes absolutely no combat whatsoever, and it is by far the quickest and most straightforward of those non-violent adventures.
There are plenty of Intermediate quests, including this one’s Holy Grail prerequisite, which not only actually include combat, but include combat that should not be taken lightly.
The main significance of this quest is that it unlocks the Knight Training Waves, which is admittedly a much greater challenge. Successful completion means unlocking high-level prayers Chivalry and Piety, which are effectively must-haves for later questing and PvMing.
Is it possible that King’s Ransom’s difficulty is overstated due to the fact that the Knight Waves was designed to be a direct follow-up? It’s certainly possible. But given the fact that the hardest part of the quest itself is picking a lock, it probably shouldn’t be that way.