Mine was Gengar. Why? Because I cheated and caught a missingno on the shore of cinbar island. It was a level 100 pokemon and when I would play people with other level 100s they would get so pissed.
Gengar was my first pokemon out and I would immediately use Sleep ray or whatever. Then he would fall asleep and my move again would be confusion. So basically he is CONFUSED and ASLEEP. Then I used Night Shade to beat him and if I had lost HP i would use dream eater which would take HP away from him and give to me.
Best pokemon ever. Usually it would take 3 of his pokemon to kill Gengar than it was was 5 v3 and I would win every time.
Always, Always, Always used to catch him or get him on any game i played, and Lapras. Lapras was such a cool Pokemon to get.
I think Mewtwo was overrated in them games as well. They make him seem like such a great Pokemon, but when i caught him on Level 70 and versed another level 70 Pokemon, i got beaten. MEWTWO ISN'T SUPPOSE TO GET BEATEN!