I don't know, I'm stuck between Drusilla and Buffy. I really think that Spike had crappy relationships! I mean, Drusilla was obsessed with Angelus and completely ignored Spike when Angelus came back. But Buffy was a HUGE bitca to him, pretty much all the time. So I guess I'll say Drusilla, then, because they were together for so long, and they were probably happy for at least a decade or two.
Maybe I should say Anya, though. Even though they were only together for one night, she was at least nice to him. 🙂