The Master Ball - The rarest Pokeball introduced in R/B/Y. It allows you to catch any Pokemon without fail, catch is, there's only one found per game.

What did you use it on? A legendary Pokemon? or maybe just a common one?

Mewtwo every time.
You can catch the legendary birds with Ultras easy enough.... you can catch Mewtwo like that as well but it just makes sense to use the MB on him... for me it did anyway.

Mewtwo lol yeah. The birds were tough, but I always went in with at least 40-50 Ultras and reset if I couldn't get them with any of those. Mewtwo was the end game mon so it only felt right to use my master on it.

I used it on one of the legendary birds, I was young when I played these games and didn't know there would be a stronger pokemon after you beat the elite 4

Now, I'd almost always use it on Mewtwo, or never use it. When I was a kid though... well ahh I can distinctively remember using it on a Ponyta.... :(

I saved it for Mewtwo.

def need it for Mewtwo

mewtwo only.

Tangela, since between being a kid and English not being my native language, I'd not realised there'd only be one Master Ball in the game- I assumed in the same way you first found Great and Ultra Balls in the wild and so on before they became available to buy, there'd be purchasable Master Balls on the shops eventually.

tentacool because as a kid the master ball remiended me of tentacool

I simply use it on Mewtwo. I would save it for a shiny, but of course Red, Blue, and Yellow don't have those.

Mewtwo because I am boring like that I guess...
I just felt like if you didn't, you were rooted.

Yeah, it seems nearly everyone went for Mewtwo haha. I myself use it for Mewtwo now, but on my 1st run of the game some years back, I used it on an Articuno as I didn't know Mewtwo existed. It was a pretty big struggle to catch it in an Ultra Ball without status effects. (I was a kid at the time.)