Read the title.
Mine was a treeko, which is now a sceptile, he's been my companion in pokemon ever since the GBA.
Pokemon sapphire was my first Pokemon game, and my most memorable, i thought through it with Sceptile, and it was a great time, i could beat anyone with him, even my friends, mostly since he became a level 100 without cheating.
He's always has been with me, by the time emerald came out a few years later, i transferred him there, and he was still great at that too, i finished the game with him, and i instantly relied on him.
It went well when i transferred him to pokemon Pearl too, and i still use him when battling on wifi.
He's probably six years old by now :D
And i'll always carry him through pokemon games, no matter what.
Currently, he's going to be transferred to Heart gold.