| The Bartender |
You've already covered three of mine, and the fourth was recently republished.
FASA's Star Trek RPG. The Last Unicorn system was okay, but a bit complicated, and the Decipher system, well, they've managed to produce 6 books or so in almost as many years?!?!?
The percentile based system that FASA used was fast and simple for new players to pick up. The rules explanations and character creation were both straightforward and gave you a good bit of control over how your character developed.
Top Secret. The original was the best. This could be redone using D20. Both D20 Modern and Spycraft cover espionage campaigns, the latter obviously concentrating on such settings.
Twilight 2000. But not as a D20 system. That would just make it another variation on D20 Dark Future type game. GDW's experience with war games showed through in the rules. Characters and vehicles were easy to lose if you were stupid or foolhardy, weapons were deadly, and healing was lengthy. All points that encouraged VERY cautious thinking with a military frame of mind.
The recently republished systems is PARANOIA XP. A great job of updating an old classic. The right frame of mind is absolutely necessary to play Paranoia, but it can be a great stress reliever.