I use the default that came with my phone, especially since I don't use my cell phone often to stay in touch with people.. usually my cell phone is used in emergency situations or whenever I'm not anywhere near an internet access spot.
On my current phone, I have it set to the default ringtone. On my old Android phone though, my ringtone was based on the boss battle theme from one of the Megaman Battle Network games (I don't remember which one it was, exactly). Also, on my slide phone, I used to have the gym leader battle music from Pokémon Gold, Silver, and Crystal. Unfortunately, this brought a lot of attention to me every time the phone rang, so I ended up changing it pretty quick. xD"
I recently changed it to one of the defaults with iOS7, but it was a song, Polish Girl by Neon Indian, for the longest time. I want the Poke Center ringtone, well more of a text tone, but I haven't really exerted any energy into actually finding it.