Half Life 1 was a revolutionary game. It brought many great new things to the table. Unfortunately it has aged rather badly. The engine is quite crap now, model animations are jerky and unnatural, and the jumping puzzles were fucking horrid (Xen anyone?).
HL2 fixed pretty much all that, it still has jumping puzzles to an extent, but because of the physics they are a quite a bit more fun to do instead of frustrating. The character animations are the best I have ever seen. Hell, they're more natural than most CG films! The 3D Engine is flexible, without being too revolutionary. I'd say Far Cry still wins when it comes to graphics and features (the gameworld is massive, with no load times after the initial start up). Doom 3 does lighting well on a technical level, but computers just can't handle that kind of implementation without looking crappy at this point of time (lol dark).
I can't really play though HL anymore, although I have finished it several times. On the other hand, there are other older games that I can still play and have fun. Most of these are older 2D titles though.
Elder Scrolls 3 - Morrowind is probably the best game I have ever played. The 3D Engine is awful technically, but it does the job, and even with the badly designed 3D engine Bethesda can make a beautiful, immersive world.
being the technological sequel to wolfenstein (which I enjoyed more)
