I'll never forget what I used my first Master Ball on: Rattata. After realizing what the Master Ball was, I forced myself to use that Rattata.
Thing is, this Rattata was really good. He almost always OHKO'd even at lower levels and when he was attacked, he suffered minimal damage. He could learn every TM/HM (much like Mew) and had absolute boss stats. I never had him evolve--clearly this Rattata was something special.
Anyways, as time went on I had other playthroughs of Pokemon. I caught other Pokemon with the Master Ball (and learned about MissingNo, and proceeded to catch everything with Master Balls haha). At first, I thought the Master Ball just made whoever was caught in it OP (Mewtwo made me think this) but as I caught more Pokemon in Master Balls, I realized this was not the case.
Never did figure out why that Rattata was so boss.
Oh, and funny side story: there's a character in Gold/Silver who gives you their phone number and uses Rattata who has the same name as me (Joey, for those of you who are so immersed in Black/White you can't remember HG/SS)!