Blew up Megaton
After emerging from Vault 101, you travel down the road a bit and discover and entire city, built from scrap, and centered around an undetonated atomic bomb. Wonderful! What’s even crazier is the mad man standing in the radiated puddle of water the bomb sits in, preaching about “Adam’s glow” and whatnot.
The rest of the city is pretty normal. However, you have one man in the bar on the far side of town that isn’t there for the drinks. Dr. Burke wants to destroy Megaton to please his boss, Tennpenny. He gives you the option and tool to re-arm the bomb, and asks you to join him at Tennpenny Tower, overlooking the city from afar.
Now, I chose to shoot this dude in the face and move on. However, I’d be lying if I didn’t say I at least made a save beforehand, and went through with his wishes. Gotta say, there was a sick pleasure to be had when you pull that lever and watch the beauty of the bomb as it consumes the city.