Polyphonic Ring Tones - 32 times more annoying than standard ringtones.
About 5 years ago I flirted with the (1970s) Battlestar Galactica theme - which really toyed with the minds of the unwashed as they recognised something faintly familiar, but couldn't place it.
Now I use the standard Nokia ringtone on my 6310i - a sound that has gone from the most common ringtone in the universe to the most obscure.
If they ever bring out a phone that let's you have all 40-odd minutes of Mike Oldfield's "Tubular Bells", I'll buy that.