Two come immediately to mind.
The first was a feral ghoul in Point Lookout. I got a head shot off, and the usual eruption of gore began, but then the body glitched and stayed in a half-exploded state for a good two minutes. Eyeballs were flying out trailing strings of goo, there were chunks of ghoul noggin hovering around them. It was hideous, but deeply funny. I was able to get my girlfriend in for a peek at it before the body finally collapsed.
The best, though, was a Super Mutant Overlord in the downtown area. I was on the roof of a smashed building, he was on a balcony very, very far away. I had managed to V.A.T.S. his missile launcher away from him and down to the street below, so he was just chilling on his balcony, occasionally bellowing and just impressing the hell out of everyone with his resoluteness. I wasn't going to waste good sniper ammo on a now functionally harmless Overlord, but I wanted him dead for the experience, and because Super Mutants annoy me. So I started plinking at him with a pistol. He was too far for more V.A.T.S. really...my disarming shot had been a minor miracle, so I just eyeballed it and plinked away. This was before I modded weapon damage, and Overlords have a comical amount of hitpoints, so it was pretty slow going. I finally wore him down to a scrap of health, and decided I wanted a V.A.T.S. kill shot. So I started lining up head shots. Miss. Miss. Miss. Miss. Miss. Miss.
Finally I hit him, and he shot straight up into the air like a rocketeer, impossibly high, a super mutant colored speck against the clouds. He did a graceful arc over several city blocks and disappeared. I found his corpse over an hour later, several streets away.