Your first Masterball.

[_knox mule_]

Lol the first time I used a masterball was on a Gligar on Silver version lol, I thought it was an extremely rare pokemon, and I didnt have any pokeballs left  

[Michi is a cat]

I used it on a Mewtwo because all else had failed...  

[Taormina]

I caught a Graveler with my first Masterball. Oh yes.  

[Prince Noir]

my first masterball was on mewtoo but i fu**ing waisted the one in my diamond i was battling dialga when somebody called i had opened the pokeball menu at that time you see i accidently put my finger on the masterball and then unconsciously i lowered my finger down (so i dont push the wrong button i thought ) then when i turned dialga was caught and my masterball was gone crying  

[Dude-LAP]

Mewtwo in Pokemon Blue and Pokemon Yellow...I was like seven years old at the time, so catching one without a master ball was impossible.