After rebattling all of them in Emerald...
Roxanne: Marill owned her, although Marill sucks and took like 10 Water Guns to kill her Nosepass
Brawly: Taillow owned him.
Wattson: This was a bit challenging. In the end, Combusken managed to kill Manectric, although barely.
Flannery: Combusken used Bulk Up. Four times. Then I swept.
Norman: Aron used Iron Defense, rendering Slaking useless. Beat him up during loafing turns, avoiding Counter. The rest of his Pokémon suck.
Winona: A bit challenging since Altaria has Earthquake. Lairon uses Iron Defense to take hits, and kills Altaria. The rest are taken care of by Manectric and Combusken.
Tate & Liza: This was difficult. The actual battle was quick, but I lost three Pokémon due to not playing careful. Aggron killed Xatu, Cacturne killed Claydol, then the others fell to Manectric and Blaziken.
Juan: I destroyed all of his Pokémon easily, then Kingdra came. Juan's strategy of Double Team and Rest is really lame, Kingdra doesn't need such dirty tricks to be a powerful Pokémon. I eventually wore him down with Cacturne's Leech Seed.
So my vote goes to either Wattson or Tate & Liza.