Hmm, that's a tough topic to answer, mostly because my views of the characters haven't really changed all that much. I still am intensely loyal to Misty (and believe me, it would need to take a LOT for me to start having even slightly negative feelings to her, much less hating her), I pretty much hated May and Dawn largely from the start, Iris I thought was far closer to a worthy successor to Misty than May and Dawn ever were, and Serena... well, she's okay I guess, though I don't really like the canon hiccup she brought about (and personally, I thought she did even LESS than May and Dawn). Tracey I was largely aloof to, Max I pretty much tolerated (ESPECIALLY compared to his sister), and Cilan, well, he's Cilan I guess, and Bonnie and Clemont I guess had some good traits to them, even if the whole bride thing of hers was a bit annoying. And the SM characters from what I gather are fine altogether. Heck, even Brock I'm regaining some long lost respect for after his brief return this saga.
If I really had to choose, though, it would probably be Brock, Team Rocket or Ash.
Ash, it's not so much I hate him or even dislike him as much as I think he was extremely poorly utilized to such an extent that they should just put him out of his misery and actually have him win a game league for once, not even caring if it ends the anime. Doesn't help that he pretty much is rendered incapable of keeping any character development with each saga.
For Team Rocket, well, technically, I hated them from the start, but it was closer to "love to hate" due to the fact that, since they're supposed to be evil, I'm supposed to hate them, never root for the bad guys or even like them. Now, my hatred is not even due to "love to hate" as it is because right now, they've been so mishandled that they're basically bad jokes of their original selves, and are basically more of a nuisance than genuinely threatening, and even in the one instance where they are depicted as genuinely threatening, they really oversold it to be ridiculous (I would have been willing to settle with them even beating Ash's rookie Pokémon at that time, especially as a bit of payback for their loss against Caterpie. Just manhandling Pikachu was overkill). Honestly, even Gaston came across as a threat by comparison to them, and I consider him among Disney's most poorly written villains largely BECAUSE of his reprise and being too open about his villainy in a very stupid manner.
And lastly, Brock (though he's only a sort-of example, as again, SM if anything redeemed him): I thought he was good during Kanto, since he had some truly redeemable and sympathetic qualities that made him far more well-rounded, like him actually having intense rage against Damian for his callous treatment of what was then his Charmander and spearheading the rescue effort of Charmander, and he was a very good mentor for Ash (Misty too, for that matter), and we even see him trying to raise Togepi as an egg. Unfortunately, starting with Johto, he started devolving into a Glenn Quagmire clone despite his humor not even being remotely appropriate for the target audience. While he did have a bit of that trait in him in Kanto, it was never THAT pronounced or episodic. Not helped was that he barely even bother to focus on his breeder goal despite Generation II actually introducing the breeding mechanic (only time it came up was the Stantler episode and the breeder convention). And AG gives even less focus to that goal (and unfortunately takes his Quagmire tendencies up to 11). Not helping matters is the writers shilling him to the extent that he makes several of the characters needlessly incompetent (like for example, Sick Daze had Misty botching soup despite the fact that, 1., she was implied to have been the one who cooked during the episodes in Viridian Forest, 2., a Cardass thing had Misty shown cooking stew without any indication she was doing a terrible job, and 3., she saved Ash, Tracey, and even Jessie's life from Stun Sporeitis disease by cooking Salveyo Weed. Plus, Brock's return in AG had Ash not knowing how to cook despite already observing Brock cook, not to mention doing very well in the Orange Islands without him, and when Pachirisu was sick, Ash fails to recognize the symptoms despite having personal experience with similar symptoms his Pikachu had twice.). And it also doesn't help that he uncharacteristically abandoned the gym even when it became especially apparent that his family was so grossly incompetent that even Misty's sisters probably were far better gym leaders than they were (like the first time, Lola nearly killed several Rock Pokemon via drowning by turning the whole thing into a swimming pool, not to mention the awful decorations, and the second time, despite Forrest clearly being in charge, she managed to have painters give it even WORSE decorations than before and even alongside Flint nearly got it burglarized while accepting plane tickets from strangers, and Forrest doesn't even ATTEMPT to fight the challengers (Ash and Brock had to), when if Brock was actually the responsible type, he'd realize he'd need to stop journeying and settle down at the gym because it would have been painfully obvious that he's the only one able to run the gym, even if he doesn't care for it.). It wasn't even until he left that he got an actual goal change (ironically, one that definitely would have made him a LOT more useful via field medics). I even feared when he returned that they'd ruin him even further, but then I discovered that, no, he actually only flirted with one girl, once, and he actually managed to be a competent trainer not seen since Kanto.