No one has mentioned Stargate SG-1 yet. Aside from the big ass CRT monitors in the control room in the first few seasons, there are relatively few things that are dated. Hell, the Simpsons references are still relevant.
Stargate will not date quickly because it was more fantasy than science fiction. The show itself is set in a specific earth time period, and the alien tech never looked like something that we would every develop on our own anyway.
X-Files has a similar advantage. Even when they dealt with "silly" things that have since been proven wrong, you could argue that we only think they are wrong and it falls in perfectly with the shows premise.