No. 2 - Dragon Slayer
The top free-to-play quest on this list probably could have been listed as number one overall here, given the fact that it is by far the hardest quest among those available for those opting not to pay for membership.
It is the ultimate goal for most free players. Defeating Elvarg for the first time is simply an unforgettable experience, and though you can only be a noob once, it is not uncommon to see players making additional accounts to experience Dragon Slayer again (or speedrunning it, as it’s one of 15 quests available in the relatively new members-only Quest Speedrunning game mode).
Just getting there is a challenge, with Melzar’s Maze being the highlight of the journey. But considering the fact that the Lesser Demon, which is the sixth and final NPC encountered in the labyrinth before securing one of the three required map pieces to Crandor, is level 82, some may be under the impression that the level 83 Elvarg may not be much tougher. But that couldn’t be further from the truth.
In hindsight, the number of hours I spent in Melzar’s Maze back in 2007 (back when what is now known as OSRS was literally just the one and only live RuneScape game) is actually crazy. I remember looking up the word "Lesser" in the dictionary, thinking that would somehow give me an advantage against the Demon itself.
Speedrunning the quest in under 20 minutes after it took you weeks to finish as a kid makes you realize that you can indeed only be a noob once.