This is a tough one because it's always the people you DON'T expect to be interesting that are!
Also, I'm surprised the Prince of Wales is not on it! He's definitely opinionated and not afraid to have a differing pov. Moreover he's now very in vogue as a visionary re global warming and organic farming expert of sorts, so I'd definitely want him at the table.
Second, there should always always be at least one pretty lady at the table. Pretty royal ladie s galore, hmm...tough one, that. Not sure on that one. Perhaps Rania would be most interesting, just judging from the fact her husband is not just a figure head in Jordan, he really rules the place. So Rania for covering a pretty and interesting lady.
Also, I think Albert of Monaco would be someone who would fit in this company as he's also interested in 'green technology' solutions, he's probably interested in a good conversation with Rania, too.
And for the fourth...I love love love Beatrix and Elizabeth and would love a conversation, but not at this table, I for some reason think they'd hinder the conversation as everyone would try to be utterly correct in their presence.
Perhaps Nicoleaos of Greece, for some <male> eye candy and I also think it's someone who'd be an interesting person hopefully.
So:
Charles
Rania
Albert of Monaco
Nikolaos of Greece