I'll leave out the ones already mentioned and add Belle-Mere. A selfish "sacrifice" made out of pride doesn't make you "OMG BEST MOM EVAR!", especially when it results in your daughters being enslaved by violence-prone xenophobes for eight years…
oh my god… you're onto something.
--- Update From New Post Merge ---
To me, the most overrated characters are the ones that, despite getting less than an episode/chapter of time in the story, are obsessed over by multitudes of fans. Characters who have done nothing, basically , who get all the praise for no discernible reason.