I don't think I ever posted such a list... maybe I've forgotten.
I Could Never Take the Place of Your Man
If I Was Your Girlfriend
I Would Die 4 U
Sometimes It Snows In April
Starfish & Coffee
I Wanna Be Your Lover
When You Were Mine
Jack U Off
the entire first half of 1999 (1999/Little Red Corvette/Delirious/Let's Pretend We're Married/D.M.S.R.)... and most of the second half while we're at it
Take Me With U
Let's Go Crazy
Condition of the Heart
Forever in My Life
Play in the Sunshine
I Wish U Heaven
I know this is a long list, but honest my instinct is to say "his entire 80's output". Including Batman, because that's got some good stuff too. The first two albums from the 70's I'm less familiar with, but I like them. I pretty much jumped ship with Graffiti Bridge... since then I've gone back and found plenty to like in his post-80's catalog, but I'm just not that familiar with it yet.
verbALs got his post in before me, but I will second his choice of the 12" versions... in most cases, they're fantastic.
Even though I highlighted 1999 in my list, if I was going to recommend a single album, I would also choose Sign o the Times. Purple Rain is pretty tough to beat too (although I've always found "Darling Nikki" to be something of a speedbump, at least on the verses).
Not a big fan of The Black Album, I find parts of it kind of embarrassing. Prince is usually awkward when he tries to come off as hard.