I'm thinking Dru on her own. First of all for consistency since she was a major villain through the entire season. Also, she was the one who actually wanted to end the world. Spike helped her with execution in the first half, because he wanted her to be happy, and in the second half, Angelus helped her execute her end the world scheme (Acathla) because he wanted Buffy to suffer, but it was Dru that was really obessesed with ending the world. Also, in season 7 when the first was posing as all the big bads, Drucilla was the one for season 2.
Although Angelus would be another really valid opinion because since he was the real emotional villain for Buffy, and the one that had to be defeated in the end, and the one she had the final fight with.
Spike tried to be the Big Bad, but ended up getting badly overshadowed.