For me, it's a tough one, but I'd have to go with Doreen as my most 'over-rated' character. Don't get me wrong, I think she is a great part of the show, but too much off her does get slightly annoying when compared to the other characters.
As for most under appreciated character, there's no doubt in my mind, it has to be Mrs Foster! Even though she was only in a few episodes, she was easily one of my favourite characters and was a lot of fun every time she appeared on the screen with a cat.