I agree, emoticons are fun, my personal favorite has gotta be :^)
I didn't know Imogen Heap sung that part of the song, ooooops.
Imogen Heap wrote the song. It's called "Hide and Seek." It was sampled later, as I'm sure you're aware. Here's a lyric video.
The layered vocal effect is from a vocoder. Basically, she sings and is able to harmonize with herself by playing other keys.
To answer the question of the thread, though, my most used would probably be the xP emoticon.