Mine was in Pokemon Stadium when i fought my Rival and won. I've only done that three times in my entire life! And the Pokemon present I got was a Squirtle! I was really happy cause the other times i got a Charmander, which was my starter. :p
So i sent it to my Red Version and am one step closer to completing the first Pokedex! ^^
In Diamond, one was when I fought Commander Saturn and his Croagunk killed my Infernape. (How was I supposed to know it wasn't a water type? It had a froggy name and was BLUE!) I barely won, but I made it!
I then tried to leave and got PWNed by a random Galactic Grunt. Boy was I embarassed.
And of course, when I fought my Champion Rival in Red. :)