It isn't too late to create an alt! Or, more practically, to peruse the lore through the Wiki.
My favorite Quest Series, you inquire? What a great question! It's honestly somewhat difficult to say, as I assess by a number of factors, though the greatest one for me is quality of the story.
I'll try to stick with completed Quest Series for this question.
The
Myreque
series is perhaps my favorite overall. I wish it had been longer, but it was almost impeccable from start to finish. The themes were great; the characters, likeable; the dialogue, wonderful. "The Lord of Vampyrium" was a sublime conclusion; "River of Blood" was an excellent epilogue. Vanstrom Klause was perhaps the overall best-written antagonist in the game.
For more lighthearted ones, the
Pirate
series is my favorite. It captured a great tone while still being serious and. A bit rushed, though!
The
Dorgeshuun
series is excellent and tends to be underrated for how tight it is. Zanik became a bit of a meme, especially in Gladz, partly because of how her character changed in the garbage that was "The Mighty Fall" and fluctuated afterwards; but she was extremely well-written within that series. The plot was also great, and everything was both earned and generally foreshadowed well.
My favorite Quests overall, independent of series, are probably "While Guthix Sleeps," "Ritual of the Mahjarrat," and "The World Wakes."
The Mahjarrat, Dragonkin, God, and Elder God storylines all had such great potential. There is some good stuff there, and even some gems; but overall, it has been entirely disappointing as far as writing goes. All of these Quests tend to be rushed, terrible in terms of characterization even if the plot is entirely sound (which isn't altogether common), and almost arbitrary in scope.