For shitz and giggles we’ve switched game systems from Twilight 2000 to the Aftermath! set of rules by Fantasy Games Unlimited. I’ve always felt that Aftermath was one of the best skills-based RPGs out there and has a combat system that goes into much greater detail than T2K does.
Basically I just got tired of the weird abstractions that T2k did for the intent of simplifying combat (e.g. The M-16 having 10 shots, a 9mm Beretta holding two, etc.). Twilight:2000 has a great set of modules but the game itself is simplistic and has limited opportunity for character advancement or improvement. Aftermath, on the other hand, has far fewer options for modules set in their post-apocalyptic setting but the system itself is great.