I think most of his instrumental compositions are under-rated, most of which are gathered on the rare All Saints collection.
As for over-rated, then for me it has to be the Diamond Dogs album, which to my ears is truly horrible. I was spurred to listen to 1984 again after comments elsewhere on this thread, but I dislike it intensely. I imagine those tracks might appeal to people who like that War Of The Worlds album, which is another that I never want to hear again, in fact I wish I'd never heard it the first time. I don't know whether it's the crass production, with those horribly naff orchestral intrusions, or something buried deep in my psyche, but Diamond Dogs is the only Bowie album that I actively dislike. I quite like Rebel Rebel within the context of a single though.
I can simply ignore Tonight, Let's Dance, Never Let Me Down and Black Tie, White Noise.