The Hoenn elite four. I know everyone's saying Kanto, but I fought them with a Starmie. That alone made them a pushover. Maybe they're significantly harder without a Starmie, but for me, they were easy to beat.
The Hoenn ones, though, bloody hell. Sure, Sidney and Phoebe are easier, but Glacia is tough. And then we have Drake. You think Lance is hard, with his two Dragonites? Well, Drake has an Altaria, two Flygons, and a Salamence, all with moves that can counter effectively anything.