Oh, I have several, but the biggest one has to be Avatar: The Last Airbender. I just...never got into it. It's good, it's well written, it's just....never caught my attention.
Every time I've sat down to watch it, it just doesn't...click, if that makes sense. It's clearly well written, the VA work is great, there's nothing jarring that takes me out of the story. It just... doesn't catch me.