I remember it well.
Sitting in the parking lot in my friends car, he mentions to me that U2 has a new song on the radio and that he had heard it the previous day.
My response was "No way, how could I have not heard of a brand new song from U2 being played on the radio" I had no idea they were even recording.
At that VERY moment, he said "yeah man, and he paused for a second and turned up the radio, and honest to God, he said "this is it!!!!" I said "No way!! really? It took me about a minute to confirm that it was U2, definitely something I had not heard from them.
So strange, but absolutely true, imagine if that were to happen to you, it was quite orgasmic, to have new U2 thrown at me from virtually nowhere.
It took me the minute to recognize Edge singing (or chanting), at the time I wasn't sure if it was Edge, but I thought it was.
Of course the song turned out to be Numb, this was a month or so before Zooropa came out.
What's funny is that they were playing this song the extremely mainstream rock station in town. Ya know, the station that AC/DC and Def Leppard fans listen to. It totally didn't fit, but at that time alt rock was infiltrating the airwaves of shit rock, so it wasn't a complete shock.
I actually heard Mofo on the radio before 'Pop' came out. Needless to say I was blown away by the sound of that song. To this day one of my top 20-25 U2 songs.
So, Numb, Mofo and Zoo Station were defintely my top 3 as far as being surprised by a sound.
I also remember hearing Discotheque on that same radio station on the way to work late one night, and I was pretty shocked by that. I'd add Lemon after hearing Zooropa broadcast in full the week before it came out. So that's my top 5.
1. Numb (because of the circumstances when I first heard it)
2. Zoo Station
3. Mofo
4. Discotheque
5. Lemon