Best SP: Fallout 3.. walking out of the Vault for the first time, I thought that was awesome.
Beating Icewind Dale for the first time, I thought I was a God. (I know that's two, but oh well)
Best MP: World of Warc.. I mean.. Throwing a Molotov on a fellow team mate in L4D while doing an Expert campaign. All four of us were running to the safe house and he got Smoked, I turned around and he ended up getting swarmed by horde. So I threw a Molotov on him and ran to the safe house. Made me laugh quite hard, even if he was abusing me via mic.