Well, I got some unpopular opinions.
From The First Album to Vogue, I've little to complain: yes, to me I'm Breathless is just a pointless album, a collection of cheesy, irritating songs - and Holiday, Angel, Dress You Up, Causing A Commotion, The Look Of Love should have been promoted with REAL videos - but she always chose the right singles to promote her albums.
I'm still not sure about Erotica as lead single: IMHO Rain could have been a better choice, and it would have mitigated the controversy around the SEX book... and why she chose Fever as fifth single whe there was something like Why's It So Hard?
Music promotion should have been longer, and [first unpopular opinion] the Above & Beyond remix of What It Feels Like For A Girl is definitely one of her most perplexing choices: I mean, not only the album version is better, but with all the great tracks in Music, why ending the brief Music promotion with such a generic, forgettable remix? WHY? What was the point of that? To show Guy Ritchie's "talent"? 
The whole American Life era: [second unpopular opinion] sorry but I DON'T GET THE HYPE. It's not a bad album, I overall like it, but the result doesn't match its political, songwriting ambitions. it totally lacks a killer track, and the title track is a mess that deserves all the bad reviews.
Beat Goes On and She's Not Me deserved a video treatment, and it could have changed the perception of Hard Candy... [third unpopular opinion] Hard Candy is a good album with an awful cover.
MDNA, Rebel Heart and Madame X: too much vocoder, too much autotune, too much trash, too many bad vocals, too many shitty collaborations, too many ordinary/cheapish productions, and so on.
Edit. I forgot her acting career... with some fine exceptions, I'm still trying to forget it.
