You know that ringtone you hear in the iPhone ads? It is the same ringtone that is not included on your iPhone! I have no idea why, either.
I just downloaded it, dragged it into iTunes, synced it onto my iPhone, and tested the ringtone. It is great. It is loud. It progressively gets louder (perhaps to not scare the crap out of you). And I am now using it as my default ringtone.
I now have two custom ringtones on my iPhone; this one, and a remix of the Imperial March that begins with a clone trooper (aka Storm Trooper) saying; “Look sir, droids.”. Both are perfect.
Source: The Missing iPhone Ringtone.