A lot of people say Dukat was a great character until he became one-dimensional.
From my perspective, he was a complex character from beginning to end who had the most dramatic arc of all. He started as a guy who was in love with himself, confident in his choices and felt just fine in his own skin. At least that's how it appeared. As the series progressed, he became a man who had lost power, lost any respect he had from his fellow Cardassians, and didn't get any "thanks" from the Bajoran people for his (in his view) kinder and gentler leadership during the occupation (I think he actually expected the Bajorans to be thankful in retrospect). Things slowly fell apart for him, and then his daughter's death drove him to madness. He never escaped his madness. Toward the end, he was truly a madman who still longed for power, still loved himself and still sought the admiration of the Bajoran people. This led him to the pah wraith cult, which gave him the power he desired, and for a time the admiration of a select few Bajorans. And then there was Sisko, who still had the office Dukat once proudly occupied. Dukat actually respected Sisko, but he did not get Sisko's respect in return, and I believe that's why he hated him so much. The writers managed to keep me sympathetic toward Gul Dukat for most of the series, even though I was often disappointed in his choices. At the very end, he was a power-hungry madman who had finally been abandoned by everyone he knew, he had no hope of gaining anyone's admiration and he knew it, and was being controlled by creatures who were equally mad = a scary combination. Yes, madmen are often one-dimensional in their actions. But the series showed us this man's very complex, very deep path to madness. The arc itself was fascinating, scary and at times uncomfortable to watch. Nothing simple about it. A great character all the way through if you ask me.