No. 1 - Desert Treasure II
Desert Treasure II is the runaway pick for number one here, and quite frankly, it might just be best if the developers don’t even try to upstage it (except with maybe The Blood Moon Rises).
Every part of this quest seems like it could be its own quest, and each boss, all of which are fightable after the quest with increased stats, introduced new mechanics into the game that revolutionized what players had come to expect out of combat encounters in quests.
And like Desert Treasure I, with its four guardians of the Diamonds of Azzanadra, simply getting to each of the “Forgotten Four” bosses is a complex journey; you don’t just show up and fight.
Whether it’s the unforgettable adventure through the Stranglewood to get to Vardorvis, the Scar to get to the Leviathan, Lassar Undercity to get to the Whisperer, or Ghorrock Prison to get to Duke Sucellus, the developers did absolutely everything they could do to make this quest the hands-down number one quest in the game.
And they succeeded.
While Duke may not necessarily be on the difficulty level of Grandmaster, post-quest Vardorvis, Leviathan, and Whisperer are largely considered to be some of the toughest bosses of any kind, not just among quest bosses, in OSRS – and that’s without getting into the Awakened versions.
And even before the post-quest stat boosts, they might very well be the top three quest bosses – pick your order – as well.
Don’t sleep on the fights with Kasonde or the wights, either. Just because they aren’t among the Forgotten Four does not mean they are easy.
You cannot afford to let your guard down against Kasonde, or chances are you will be hit with an explosion and/or poison attack, and the wight fights in the Ancient Vault against the Forsaken Assassin, Ketla the Unworthy, Kasonde the Craven, and Persten the Deceitful can all be extremely punishing.
It would be smart to prepare for this encounter just like you would for any of the Forgotten Four; the quest is not over just because you’ve prevailed over the main bosses, and you need to beat the four wights in a single instance to succeed.