For me it's definitely between Sasuke, Madara, and Kabuto. Sasuke's descent into madness from feeling of revenge was awesomely written, although his more recent decisions to protect the Leaf pretty much turned his characterization in a complete 180. Madara? Refer to my previous thread on his motives in leaving Konohagakure to assess my opinion of him.
Kabuto quite frankly takes the cake for me, however; growing up he lacked something that most people take for granted - an identity. He didn't even know who he was deep inside. He used power as a way of gauging how far he was towards gaining an identity; the stronger he became, the more and more his cocky, arrogant attitude faded away to show insanity. In his 'divine power', he believed he had evolved past the pitiful human role, and become something more. As just said, his self-sure facade shown in Part 1 and mid Part 2 was simply a means of hiding his true confusion inside: who was he?