The one who will bring balance to the Force....
GL has stated that this refers to Anakin's redemption and destroying the Emperor. Which leaves us with one known good Jedi and one with potential.
That aside
could he just as easily restored the balance by wiping out the Jedi Order? Just something to amuse your brain. I mean by Ep2 we know something is wrong with the Jedi. Mace says something about admitting their powers are diminished. They don't notice Palpatine under their noses! (Assuming he is Sidious, not a proven fact yet, but that's another thread)
Sith and Dark Jedi are not the same thing. It is possible to be one and not the other. It just happens that in the movies the only Dark Jedi we see are Sith.