Well now, that context actually changes my answer. "When Doves Cry" is it from a musicology perspective, but if you're introducing someone to Prince and one taste has to represent the whole buffet, I think it's "1999."
The opening chords, the musical intricacy, the very Prince vocal style, the apoclyptic-ness, the percussion track, the funk rhythm, it's the whole package.
Plus, it captures the "one minute to midnight" aspect of life in the 80s that is so impossible to explain to people who didn't live it.