No. 3 - Swan Song
There really isn’t much to say about Swan Song that could justify making it a Master quest, making it an easy pick to include on this list.
Back when the quest was released in 2006, the battle with the Sea Troll Queen was probably considered pretty state-of-the-art, and the onslaught of Sea Trolls you had to beat to get there presented a unique wave-type approach that not many quests utilized at the time.
Still, that should only make it borderline Experienced, and nowhere near Master. If the quest were released today, that’s exactly what it would be classified as.
This is the one and only Wise Old Man quest in the game, and I’ve long advocated for the addition of RuneScape 3 quest Love Story to change that. After all, the Wise Old Man is literally the individual to whom you must speak to claim the prestigious Quest Point Cape.
Love Story is far more challenging than Swan Song (in RuneScape 3) and is still listed as Master quest, which further attests to the fact that Swan Song should be at least one rung lower on the quest difficulty ladder in OSRS.