Dukat was easily the most complicated villain in the history of Trek and arguably the best one just by his sheer force of personality and will. What made him more than just the cliche villain of the weak was he did have admirable qualities. He was patriotic to the core, a dedicated family man(when he wasn't cheating anyways), certainly he was loyal to his friends if Damar is any indication, and he was polite when he wanted to be.
Certainly this led to the grey area during seasons 3-5 when you were almost wondering if Dukat had come over to the good guys or not. He hadn't really changed that much at all, just his objectives and goals corresponded with the good guys and so he realized that and would work with them when required.
He was quite skillful in overcoming adversity and you never could keep him down for very long. His obsession with Bajor and by extension Sisko was both his greatest strength and greatest weakness. It gave him a driving force at times, when all the world was arrayed against him, but in the end it did pave his downfall into madness.
In the end he did covet power and control and had a self-love that he seemed to want everyone to share. His actions during the occupation gave a glimpse at this, and even joining the Dominion and the Pagh-Wraiths wasn't so much about those causes as much as it was him placing himself back on top again.
Marc Aliamo was superb in portraying him and it wouldn't have been the same without him, that is for certain.