Who would make up your best team for the Master League?

Most important imo is the movepool.

Master league is pretty much dominated by legendaries and pseudo-legendaries, the pokemon with the highest stat products. Groudon, Giratina, Latios, Latias, Lugia, legacy Dragonite, Kyogre, Mewtwo, Metagross etc. There aren't that many pokemon that are useable if your opponents have access to the strong options, a single Giratina or Latios could sweep an entire opposing team of weaker pokemon.