Bjork was originally on there, and while I love her, I chose based on who I think would spark up interesting discussions. She'd be all coy/quiet for most of the meal, then really awkwardly bring up the melting ice caps or something and everyone would go quiet.
Tori Amos would just piss everybody off. I can imagine she's one of those people at dinner who like announces that she's about to tell a story/anecdote/joke and then spends ages elaborately explaining it. Comes off as pompous or whatever...