On the topic of Ace pokemon:
Kats has several that can qualify, depending on the circumstances, but there are two that generally fit the bill: Jaws, his over-sized feraligatr and official starter, and Molly, his granbull.
Leone's ace is her tyranitar, Bulk. While not expertly trained, he's been with her since the start of her journey, alongside her typhlosion and slowking, and is indesputably her strongest pokemon.
Iori's ace is Dux, their Porygon-Z. A little more stable than most of it's kind and frighteningly powerful, it's also on the same mental wavelength as Iori and sometimes has to act as their common sense.
Michael's ace is undoubtedly his umbreon, Havoc. Having been a force of chaos as an eevee, she's grown to be a mature assistant and a steadfast guardian of the pokemon Michael cares for.
Jupiter has her skuntank, known to most as Vicious. A registered therapy pokemon, Vicious is very protective of Jupiter, thus her more well-known 'false' nickname that she's very prideful over. Her ACTUAL nickname, which only Jupiter is allowed to call her, is Miss Stinky.
