- Rome Total War/BI, plus user mods

- Galactic Civilizations 2

- A chess game of some kind, if we're really talking desert island

- Neverwinter Nights with all the official expansion packs, and a selection of the best user modules

- Orbiter with all the user mods

That last one, if you haven't heard of it, is a realistic space sim based on the Shuttle and previous platforms. It's complicated and really hard to fly with full realism settings. I think I'd need to be on a desert island to get up enough motivation to learn how to fly it well.