On my old cell phone I used the first level of Sonic the Hedgehog 2
http://www.vgmusic.com/music/console..._Hill_zone.mid
With my newer phone, I just left everything how it was because the ringer isn't loud enough with MIDIs for some reason.
By the way, this site is awesome for cell phone ring tones - if you like videogames. Most cell phones will play MIDI files too.
VGMusic - 28,365 Game Music MIDI files