I use the *Dies Irae *from Mozart’s Requiem for calls from my army reserve unit.
Tried to get as close as possible to it: this is my current general ringtone, for calls that don’t have a specific one. I think it’s funny, because I’m a prosecutor.
By the way: to link to a specific point in a YouTube video, end it with “#t=0m00s”, putting the correct numbers in place of the 0s, as I see Lilyof theValley did. You can also strip off the “&feature=related” bit.
Mine’s just a Northern Cardinal’s song. I used to leave it on vibrate but I couldn’t hear it in my bag. If I had a song as a ringtone I’d probably pick something metal. 
My ringtone is set to use Christian Bale’s Italian song remix. Well, the first 10 seconds or so.
I usually leave the phone on vibrate when I am at work or someplace it could disturb anyone.
My ringtone is “Wish You Were Here” by Pink Floyd.
“How I wish, how I wish you were here
We’re just two lost souls swimming in a fishbowl.”
My ringtone is the “doorbell” sound from Star Trek The Next Generation.
The opening bars of London Calling. Occasionally someone gets the joke. It would be better if I got calls from London, but a man can dream.
My default ringtone is, and always has been, Ace Of Spades by Motörhead.
But then I make custom ones for nearly everyone who calls me often, so I can know who it is without looking.
I love that song so much. Haven’t heard it in years and years, though.
I have the siren from Kill Bill (the Ironsides theme). It is the most annoying sound ever.
I don’t have anythign that creative. Just the first verse of “Wanted Man” by Johnny Cash. Proably millions of other people have it.
My wife has a recording of a bored-sounding guy saying “Ring, ring, ring (sigh)”.
One of the office fussbudgets has a ringtone that says
“Are you there? Are you there?”
“Are you there? Are you there?”
“Are you there? Are you there?”
Some days I’m like, “Yeah, I’m here, what the fuck do you want?!”
Mine is the Peter motif from Peter and the Wolf.