I think I'd have to pick a song from 3 different eras:

Joy in Repition - for the pure funk.
Friend/Lover/Sister/Mother/Wife - for the beautiful ballad and lyrics.
Dreamer - for the awesome guitar.

Let's Work
Computer Blue (hallway speech)
Crucial

Adore
When Doves Cry
The Word

Kiss (Perhaps the ultimate pop song)
When You Were Mine (the melodies and infectious grooves are killer)
Purple Rain (one of the best ballads ever written)

if I could pick a 4th, Erotic City would go in there.