Come Fly with Me, Fallout New Vegas.
The Fallout Series, in general, has great side quests, but Come Fly With Me is one of my favourites for the way it takes something that seemed known (Ghouls) and added twists (A human who thought they were a ghoul? A Ghoul that wants to fly to the stars and escape?) and encourages exploration of a location that may not have otherwise happened.
Love it.
Edit to add:
I didn't want to expand originally to prevent spoilers, but for me, the first time I found this quest had aligned with Novac and Boone, one of my favorite Fallout New Vegas companions (and even Fallout companions of all time, really). I had just gotten the charisma high enough to get his quest and learn, but I didn't know where to start looking, and at the time, I didn't know that side quest could be done entirely in Novac, so I had inadvertently hiked all the way to the plant, and discovered the ghouls by happy accident. When I go back to replay Fallout new Vegas, it's one of the side quests I seek out if only to get to talk to Jason and Chris again, and explore the options of the Questline. I always help the ghouls to fly though.
