Articuno, way back in Red. This was before I got the internet. In fact I only knew about Pokemon because it was mentioned in a cheat book and I thought it was a new Final Fantasy game. So I knew nothing of Mewtwo, I thought that Articuno being the only pokemon who was visible outside of battle meant he was the strongest in the game. After numerous failed attempts I finally used my one and only Master Ball on it.
Funny story, I then discovered the vastly superior Mewtwo, had it at optimum catching status, threw all my Ultra Balls at it to no avail, it then used Recover, I was out of PP and on my last Pokemon, do or die time, I threw the last ball I had on me, a lowly Poke Ball, and caught him. Of course now I understand it was just sheer luck but back then I was strutting around the house like a legend. I caught Mewtwo with a regular pokeball, said I. Bow down beneath my mighty gaming prowess.
Yeah, I was a twat back then. Haven't changed much.