What did you use your Master Ball on?

[Amore]

Ultra Ball matches Beedrill as its also Yellow & Black I would also consider Luxray as a contender for it.

Yeah, but Weedle and Kakuna are so easy to catch I don't have to bother! You're right though, there are plenty of Pokémon that look decent in it…but a lot are in the later games.
Shiny Luxray and Ultra Ball are the brilliant match.

 

[Adamabbafan]

I use Quick Balls, Luxury Balls, Premier Balls, Great Balls, Timer Balls, Dusk Balls & Ultra Balls on Legendary Pokemon my absolute pet hate is seeing any Legendary Pokemon caught in the regular Poke Ball.

I am a firm believer in using the Master Ball on Legendary Pokemon only but I mostly use Ultra Balls if I want to save the Master Ball for an important Legendary.

I caught Giratina & Cresselia in a Luxury Ball in Alpha Sapphire, Moltres in a Dive Ball in X (purely by accident), Ho-Oh & Groudon in a Premier Ball in Omega Ruby, Kyogre in a Dusk Ball in Alpha Sapphire, Suicune in a Net Ball in Colosseum, Zapdos in a Great Ball, Articuno in a Nest Ball and Moltres in a Timer Ball in XD: Gale of Darkness and my only critical capture on a Legendary was Koraidon in a Quick Ball in Scarlet.

 

[CoolKid575]

I haven't used it yet. I'll probably save it for one of the Ruinous Quartet or Miraidon.

 

[Angel-of-Light-Kelly]

I mostly save Master Balls for shiny hunting, when I really need to catch it. The Legendaries, you can just save in front of them. I kept my master ball in Gen 7 for wormhole shiny legendary hunting (still haven't used yet), the one in 8, nothing yet. 9 I haven't used, though maybe if I find a shiny Voltorb...

 

[Dr. Seuss]

The point of using the Master Ball is to catch the main legendary, so I used it for Miraidon.

 

[PokeDave854]

I just tried doing a solo run vs Greninja with my lvl 100 Gastrodon and got destroyed. I only fainted once, which is good.. but I had a hard time dealing any real damage to it. We need something that can hit it harder.

 

[Diamond 97]

Nothing. Quick, Repeat, and Timer Balls are plenty powerful enough for every Pokemon and are way more plentiful.