It can be a main or side quest. For me, There's a lot of missions from Fallout New Vegas that are my all-time fav missions, but if I had to choose one, it would be Come Fly With Me. And yes. I messed up the launch. Best. Side quest. Ever. My runner up would be The Power Of The Atom from Fallout 3. Reason why is because it gave you one MASSIVE choice so early into the game that can determine the fate of an entire town. What i loved about it is that it really made you ask yourself if you were gonna be the good guy or the bad guy, So if you screwed up in your choice, you've done one BAD decision you're gonna have to live with for the rest of the game. I saved Megaton myself since I was doing a good walkthrough of the game.