Mine is Ace of Spades by Motorhead. Two reasons:
1. I have a good association with it from The Young Ones.
2. There’s something funny about hearing a song that could be classified as “death metal” in that cheesy 1980s Casio tone that my phone offers.
What’s yours?
Mostly Modern Media 
I think ringtones are evil and in most cases, annoying. I keep my phone on vibrate 98% of the time, and when that isn’t possible I use the sound of…a ringing phone (which I had to SEEK OUT ON THE INTERNET because cel phones don’t just ring anymore. They come with incredibly obnoxious rings that no human being in their right mind would use, becuse they want you to buy ringtones. I could rant about this all day.)
That said, a monophonic 8-bit Ace of Spades is probably pretty damn cool.