Mine currently is a MIDI version of the intro to "Newborn" by Muse. I downloaded it from some web ringtone archive I found through Google. I've been using this as my primary ringtone for about 2 years.
I hate MP3 ringtones. The fidelity of mobile phone loudspeakers is horrible, and I just think MP3 ringtones are ultra tacky, especially since people who like this type of ringtone tend to choose urban/hip hop.
I also like simple "ring ring" type ringtones but I never really hated well-done, recognizable, catchy multitimbral midi tunes. Trouble is, "well-done" is hard to find.