I don't despise Ash, but I find it amazing that this dude could be so stupid for so long. It's almost no wonder he dropped out of school. Or maybe it's because of it. It appears as if I've found a vicious cycle.
I also never really liked any overly bland trainer of the day, especially if mr./ms. white bread got more screen time than their Pokemon of the day, who was equally as tasteless as water, but was at least more visually interesting, considering they defied the homogeneity of the human character's designs.
I dunno. The thing about the Pokemon anime is that everyone is either too nice/soft to be completed hated (unless this has changed, but even Giovanni, with his self interested desire of power at any cost, has moments where you don't really hate the guy 100%), or too dull to feel any kind of arousal for. This is certainly where the manga excelled, I feel. Otherwise, it seems hatred for a character is due to a personal pet peeve, whether that's voice acting or a personality type that doesn't jibe with a viewer.