The whole prophecy re: balance in the Force is problematic because (a) it comes out of nowhere after the original trilogy, but more problematic in the context of the story because (b) it is not introduced at the start of Episode I. The (astonishingly dull – trade routes? really? c’mon!) opening scroll leads us to believe all is well in the Jedi world. It’s not until Anakin is introduced that the prophecy is mentioned, and, sorry, that’s too late. There should have been discussion early on about “Master, have you noticed the Force is out of balance?” “Yes, my young padawan, but remember the prophecy…” Blah blah blah. You can’t shoot a gun in Act II that you haven’t put on the mantelpiece in Act I.
That being said, since we are stuck with some prophecy of someone bringing balance to the force, the theory I like best is that the Promised One is Luke, who manages to balance detachment with emoton, something the Jedi we see in the prequel trilogy never manage to do (as ol whatersname alludes to above).