What should you use master ball to catch in fire red?

Answer 1

All legendary pokémon can be caught with Ultra Balls, but your team needs to be strong in order to get them weak enough for a decent capture rate. Because the birds (Articuno, Zapdos, Moltres) and Mewtwo sit in one place, you can save before the fight and keep trying until you get it right.

That being the case, I would use the Master Ball on the roaming pokémon (Suicune, Entei, or Raikou) that appears after the elite four has been beaten. These three legendaries travel around the map quickly and will flee from battle, making them rather time-consuming to hunt. Instead of preparing a team to nab it, simply using the Master Ball when it shows up will guarantee a capture and save a lot of time in the long run. Remember that only one of the three will show up in your game. Which one it is depends on which pokémon you started with: Entei if you picked Bulbasaur, Suicune if you picked Charmander, or Raikou if you picked Squirtle.