It's hard to choose, as it is one of my fave M. albums, but I'll pick Bad Girl. It is my fave MV of hers and the song is deeply beautiful.
Words would be my second choice, I absolutely love how it slows down during the bridge: ''friends they tried to warn me about you...''.
And, of course: Erotica, one of my fave songs ever, so sensual and I love how her voice sounds.
Bye Bye Baby, DAD, TOH, Rain, WISH, ITL and Secret Garden are standouts too.