THE EVOLUTION OF THE TWILIGHT: 2000 LINE
     
Twilight: 2000 First Edition (released in 1984) was
much too math-heavy for my taste!

Although a fun game, the vehicle-combat rules were almost impossible to run quickly and many rules aspects weren't in place yet (recoil rules, shooting while running, aimed shots vs. area burst, etc.). And the history portion in the beginning of the book ran from 1995-2000.
     
Twilight: 2000 Second Edition (released in 1990) was a complete overhaul of the system, simplifying much of it (for the better, IMO) and changing the game from % rolls to d10 rolls - I was amazed at how quickly I was able to create a character after the marathon-session chargen of the First Edition. And the history in the beginning of the book changed to 1989-2000, so at least the first year of the history was already familiar to readers (it mentioned the Panama Invasion and the Gulf War).
     
Twilight: 2000 Version 2.2 (released in 1993) was the final, and IMO the best, version of T:2000 to date (unfortunately, GDW went out-of-business three years later

). While not the overhaul that came between the first two editions, it polished up many of the Second Edition's rought edges, and changed the system from d10 rolls to d20 rolls. This time around, the writers of the history in the beginning of the book had the benefit of hindsight... 1989-1991 went exactly as it did in real life, while in the game's history, KGB's Alpha Team obeyed the Soviet coup leaders' orders in August, 1991 and stormed the Russian White House, killing Boris Yeltsin and his fellow rebels and restoring a Stalinist rule on the Soviet Union. Surprisingly, though, most of the rest of the game history ran similarly to the Second Edition's version - so basically, they took a point in history's past (by the time Version 2.2 hit stores), changed it, and thus created a "what-if" world whose history no longer needs revising but instead simply becomes a different dimension (similiar to the "What if the Germans had won WWII?" or "What if the Roman Empire had never fallen?" type of settings).
     
Merc: 2000,
Dark Conspiracy and
Cadillacs & Dinosaurs all used the Second Edition ruleset (as they were released prior to Version 2.2), although
Traveller: The New Era used the Version 2.2 ruleset. However, while
2300AD was set in the
Twilight: 2000 universe (just 300 years later), it used its own completely different ruleset (evolved from its previous incarnation,
Traveller: 2300, which contrary to what some initially believed, had nothing to do with the
Traveller continuity).
      On a side-note: until GDW developed one house-rule system for all their games, it was annoying how each of their RPG's had great settings but different game-systems from one another.
      I hope this helped!
-G