Ten desert island RPGs. Ten is already very generous, so I'll stick to core books only.
- D&D Rules Cyclopedia: For playing D&D, which is very hard to do with another system.
- Street Fighter: The Storytelling Game: Man, I can't tell you how much I love this game. Seven years worth of frequent play hasn't dulled my enthusiasm for it. It's the system for gonzo modern martial arts / adventure gaming. Only Ninjas and Superspies could even begin to lay a claim to being able to cover the same ground.
- The Fantasy Trip: For tactical fantasy RPG combat with a sword and sorcery vibe, look no further.
- Heavy Gear: The system gives you wargaming, roleplaying, and vehicle construction in an elegant package, plus a great setting. I'd also use this book (without modification) if I wanted to run a GI Joe game.
- Call of Cthulhu: Good for CoC style games, of course. Also for the system, which would work for a variety of things.
- Warhammer Fantasy Roleplay: For the old world, for the career system, for critical hits, and for darkly humourous gaming.
- Castle Falkenstein: Every time I've played it, it's been a full success. And I'm not even a steampunk afficianado.
- TMNT: For the mutants, for the ninjas.
- Poison'd: I'm on a desert island RPG, so the appeal of pirates is obvious.
- Traveller: This has a lot of range.
Nightmare mode? It'd be Street Fighter, Heavy Gear, or D&D. But I'd hate to have to make that choice.