[Bull of Heaven]

The first Pokemon I ever got to level 100 was an Alakazam in Silver. I still remember how proud I was that it could solo Red. At some point my brother borrowed it to help him beat some opponent (which he had done at least once before), then forgot he had it and restarted his game.

I used an Alakazam in my first SoulSilver run in its memory, but I chose not to replicate the Psychic/Fire Punch/Recover/Flash moveset. I also kind of remember my surprise at how much damage an Abra could do with a Fire Punch TM back in the day.

Edit: Also finding a shiny Donphan in Silver, only to discover that my box was full.