It depends. If we count rematches, my vote definitely goes to Flint. Hands down. In Platinum, he packs a huge team that can leave holes everywhere and without strategy, you're not gonna get out of the battle too easily. He had good coverage and thought about countering what would be checks to most of his team. If Sunny Day had been set up by Rapidash, Magmortar was free to come in and fire off Solar Beams whenever it pleased. If you somehow managed to get past Magmortar, you would still have to worry about the quick-paced Infernape that can just smash through even the most well-prepared Pokemon. Flint's team is dangerous. Especially when they're 15 levels higher then my most well-trained Pokemon. (I don't like grinding okay ;~;)
As for the others, I've never found Aaron that challenging. Then again about three quarters of my team resisted bug and had some sort of move to fight back against the critters. Bertha was fairly easy as ground does have quite a few weaknesses to hinder it. Plus they were all ridiculously slow, it wasn't too hard to get a hit on them and finish them all off. Rhyperior to my surprise was rather bulky, but even that couldn't withstand a quick Surf. Lucian was arguably rather challenging, but I could easily get past with a dark-type and Staraptor flying out of Gallade's reach. :p