I'm not counting hearing songs off the album, coz that's a deliberate listening experience. I'm talking about that chance encounter with a new P song that made your insides get up, get into it and get involved!
4 me it's:

LRC (radio in my teen bedroom)
WDC (ditto)
Kiss (my brothers had heard it and had been going on about it for a few days, but I hadn't heard it yet, so when it came on the radio one afternoon after school one of them ran and turned up the stereo loud on the first few notes)
SOTT (It was the first song on my clock-radio one morning and made me jump str8 out of bed!)
D&P (I was peeling potatoes in the kitchen at a summer camp, when I heard the first few notes, and thought "This reminds me of Romance 1600. Must b Prince, but I hope not!!")
Dolphin (I was travelling in South America and we went to this pancake place in Quito, Equador for my birthday breakfast. I got there first, and they had a big screen playing MTV. I hadn't heard any new music for months and then this came on!!!)
TRC - nor exactly a single, but I heard it on an evening radio show on the mainstream station here in Israel and freaked!

That list was longer than I had expected! What about u?

1999-friends bedroom

I Wanna Be Your Lover- working in a shop

Contorversy-working in a shop

WDC- in my bedroom searching frantically for it on radio

Raspberry B--in a friends truck, helping them move

Kiss..in my bedroom

Batdance- in my room again

My Name Is Prince--on car on alternative radio station