I'd have to say that the 'Ichigo vs Ulquiorra' fight was on par with 'Kenpachi vs Nnoitra'. I just think that Kenpachi and Nnoitra are so goddamn bad ass. [as well as being plain fucking hot]
'Bad guy' being a general term; I rather like Kakuzu, Hidan, Kisame, Zetsu, Suigetsu, Nnoitra, Grimmjow, Ulquiorra, Harribel, Starrk & Linette, Gin [ichimaru], Vincent Law [from Ergo Proxy] and a couple more that I can't think of at the moment.