Ok, I'm gonna be the second - I guess - unpopular one. For me, it is a pain, how overrated and over-mistified Confessions is. It was and is a great album and it was a very strong era speaking of image and style. But other than that it is simply high quality retrodisco, nothing else... so for me it is the most overrated one. As for underrated - I guess it is up to anybody, and it depends on when you become a fan. My fandom started in 1996 with Evita, and for me, the most underrated record is American Life, but yeah, for today Music is also a bit forgetten, compared to Ray of Light e.g. But it is only a massive private opinion, nothing more - of course I love all her legacy, some more, some a bit less (yeah, poor MDNA, I'm sorry...)