That's a tough one Ray. My answer depends on the purpose of the dinner! For girly gossip and probably a good laugh then Nigella. For flirtation then it's Thomas Keller (my culinary crush). But the person I would most like to have met is the late Jane Grigson - so knowledgeable, erudite, witty and apparently also extremely nice.
You didn't tell us who your fantasy dinner would be with.