Half Life series have been the pinnacle of FPS genre for years. The original game, Half Life 2 and the rest of the games in the series, all have introduced something new to the gaming world. Some of which have become the defining features of a story driven FPS game.

Half Life 2 introduced the state of the art physics engine, advanced AI and the powerful source engine.
I have played a lot of FPS games but, nothing feels like half life 2 even when it released in 2004. Sure, the gunplay and platforming have been improved a lot in modern games but, for the times of 2004 this must have been mind boggling.

I am playing each half life game in release order and this is a hell of a ride. Seeing the advances each game made over the previous iteration makes the experience worthwhile and super enjoyable. I hope the series reach to the conclusion with Half Life 3 and Valve will do justice to Half Life and it's community.