For me it`s probably Harry Potter. I don`t think it deserves all the popularity it gets. The plot is pretty generic, the characters are not particularly interesting (at least to me), there is almost no moral ambiuity and I don`t find the villains cool (Voldemort, Bellatrix and Umbridge) and it is too childish for me to enjoy. I tried to like it because I know a lot of people who love it, but it could never get the appeal.
Another franchise I think is overrated is Avatar: The Last Airbender. The only character I like from there is Zuko. Other than that, I find all the other charaters annoying. I also think that the show is not as mature as people give it credit for. There are too many moments that I find childish for this series to be mature, even though it explores some more mature themes.