Whilst normally I'm one for the rule of "Don't Bump Old Threads", with Half Life being my absolute favourite game series, I need to contribute to this.
Half Life is quite possibly the single most influential and revolutionary FPS games of all time, it was one of the first games to feature prominent use of the Havok Physics engine, to make use of real death animation, rather than Ragdoll Physics and at the time, was so far ahead of anything else, it was outstanding.
Half Life 2 and the subsequent episodes were extra greatness to the Half Life franchise, just adding to the superb storytelling, excellent graphics, wonderful level design and enjoyable puzzles, it was a game that not only drew you into the experience, but also made you think before you acted.
It was also a game that brought strategy into play, how much ammo do you have left? Which weapon is appropriate for this situation? Am I better off using my crowbar for now, rather than my guns? And so on.
My only gripe with the game was that the vehicle sections were a pile of arse, and occupying a vehicle at times felt like steering a shopping trolley weighted down by a hippo, but everything else makes up for that one, minor detail, the maps are huge and massively detailed, you're never likely to find a more revolutionary and brilliantly designed game to be honest.
'Nuff said.