I think all these fights can be considered the greatest in their own way.
Bonnar Giffin because of the historical impact it generated throughout the ufc and MMA as a whole.
Rua and Henderson as an prime example of heart and leaving EVERYTHING you have in the octagon.
Jones vs Gustafson (Much like Gus vs DC) can be declared the greatest because these were beautiful hard fought contests at the peak of skill within the light heavyweight decision.
Thus, it is truly dependent on context and which factor you consider more "deserving" to be the greatest.