No. 3 - Monkey Madness
As a free-to-play player, the best weapon in Old School RuneScape is unequivocally the Rune Scimitar. So naturally, the fact that completion of Monkey Madness unlocks the ability to wield the Dragon Scimitar, a huge upgrade from a stat standpoint, makes it a highly desirable quest to do for new members.
Monkey Madness is a lengthy adventure that demands players not let their guard down as they traverse Ape Atoll for the first time and aim to help Narnode Shareen, king of the iconic Tree Gnome Stronghold, track down his 10th Squad military unit, which has mysteriously gone missing.
Traversing a winding Zooknock's Tunnel, disguising oneself as various monkeys, and avoiding having one’s cover blown are all vital elements to this fan-favorite journey.
As the third Master quest (the second was Regicide, the sequel to Underground Pass which is actually now an Experienced quest), it was widely believed to have overtaken Legends’ Quest as the toughest quest in the game upon release.
Who else remembers using the Gnome trick, safespotting the Jungle Demon while allowing the Gnomes to do most of the damage during the boss encounter?