The black uniforms they came to Unova with. Now that I've seen that the Trio HAVENT regressed into goofballs once their switched back to their white outfits and have remained competent, I'm even more annoyed about Episode 23 costume change. Their apparently continued role as competent villains, especially in this episode, would have suited their black uniforms very well.
Heck, in the early Unova episodes the TRio tried to steal Pikachu
in their new outfits several times and there wasn't a confusion of interests or intent at all, so i really don't know why the animators chose to hype up their uniform change before the series aired only for that to be a temporary gimmick while maintaining their new role.