If "Purple Rain" is Prince's career defining album of the 80"s (by majority of critics although I would put 1999 and Sign 'O' the Times near the top as well), then what would be his defining album of the 90's and present decade.(You can add the 70's if you want to even though there was only 2 albums). Well, my choice would be this: 90's-Diamonds and Pearls (with Gold a close second). 2000's-3121 by far. What do you think?

Diamonds and Pearls

The Rainbow Children

D&P--but the Truth may be my favorite of the 90's, or possibly

Musicology, if only because there was a tour, singles, videos, album, and Grammy award that all got a little bit of notice and served to define his career for a new decade. It updated his image in people's minds who hadn't seen Prince in a long time. TRC is my personal favorite, though.
[Edited 5/1/07 14:44pm]

Well, if you're just talking in a commercial sense then D&P for the 90's and 3121, so far, for the 2000's. I'm tempted to say Musicology but the concert giveaways make that album seem more popular than it really is and 3121 and Black Sweat charted higher, IIRC.