Good question, Sweats. I love these hypothetical scenarios, so I'll give the thought it deserves.

In no particular order:
Guest 1) Charles Darwin. No brainer. My hero and a constant source of inspiration. Would love to hear about Natural Selection from the horses mouth, as it were. He had an encyclopedic mind for Natural History and I'd love to pick those brains for tidbits and original thoughts on the simplest of matters and the most lowliest of creatures. He would also have all those salty tales of adventure on the high seas and a few on more terrestrial wildernesses. Also, I could talk to him about how he coped with a devout Christian wife when he was clearly an atheist. The dynamic of their relationship would be another conversational string to his bow.
Guest 2) JBS Haldane. Picked to spark off Darwin. One of the founders of the New Synthesis. Haldane could explain the wonders of genetics to Darwin (as Mendel's work was unknown to Darwin despite their being contemporaneous!), and how this has strengthened immeasurably his 'Theory'. A polymath, Haldane would not be without other conversation. The talk between Darwin and Haldane alone would be worth an Eternity of other dialogue.
3) George Orwell. One of my favourite writers. A man who was, like Swift, economical with the language without losing any power from his prose. A man who was born rich, yet spent time living as a vagrant to understand those less fortunate than himself. A man who started out as an adamant proponent of communism -- he fought in the Spanish Civil War and received a bullet to the neck for his trouble! -- but, through contact with its results, eventually became one of its fiercest detractors (see Animal Farm). He would be a joy to talk with for all his experiences and his thoughts on class and society.
4) Carl Sagan. Another one of my scientific heroes. Physicist, Cosmologist, writer, popularizer of science, and a fierce speaker for of liberal rights and politics. He could talk about science from another perspective: the world of atoms, quantum mechanics as well as gas giants, black holes and nebulae. He would introduce another strand of conversation by discussing the big bang, stellar evolution, etc. He could also discuss political ideas with Orwell and the others.
5) Terry Pratchett. My favourite contemporary author. He loves science and technology but also adores history and the history of science. His books also contain political ideology and satirical characterization. He could comfortably discuss most of the ideas offered by the others and add a much needed element of humour -- besides my dick and fart jokes, of course!
Tough question really, Sweats. My choices reflect two of my passions: science and literature. There are other people I'd love to talk to but those (above) would probably be my top 5. Hopefully, you won't have too many p*sstake answers -- a few are always needed and welcomed!
