My 2 favorite kills are from a few years ago when I used to play World of Warcraft (back when the level cap was still 60).
I had just gotten my Whirlwind Axe for my level 35 warrior, and I was running around solo with my new overpowered (for my level) weapon. I came up to a level ?? Horde thief (I played Alliance) chasing a level 34 priest around the map. I attacked him as they were running around, and, after a short while, I'd done enough damage that he ignored the priest and turned on me. At that moment, I used Retaliation for the first time ever, and absolutely destroyed him. I know it didn't take much skill on my part, but it was extremely satisfying at the time.
My absolute favorite kill of all time though wasn't even mine, technically. I had gotten my wife to play WoW with me (she was pretty terrible, but she had fun). She played a thief; I played a fully defensive shield warrior (I had a separate warrior just for playing with her). Anyway, we were running around Stranglethorn Vale together, both of us around level 35 or 36, when I got Sapped by level ?? Horde thief (not the same one from my first example). He went on to kill my wife's thief in 1 hit while I sat there stunned, then he turned on me. I fought what was easily the longest 1v1 pvp battle I ever fought using every block skill I had including Shield Wall, and, while I held my own fairly well, I was losing the battle. The thing was, I held out long enough for my wife to run all the way back from the graveyard, stealth, and Ambush the guy for the kill. Not only was that my favorite kill ever, it was the best time I've ever had playing a video game.