Playing the Half-Life series in release order is generally recommended to fully appreciate the evolution of the games and the story. Here's the recommended order:

1.Half-Life (1998): Start with the original game to get introduced to the story and characters.

2.Half-Life: Opposing Force (1999): This expansion pack puts you in the shoes of a soldier during the events of the first game.

3.Half-Life: Blue Shift (2001): Another expansion pack, this time you play as a security guard during the Black Mesa incident.

4.Half-Life 2 (2004): After finishing the original game and its expansions, move on to the highly acclaimed sequel which takes place several years after the events of the first game.

5.Half-Life 2: Episode One (2006) and Half-Life 2: Episode Two (2007): These are episodic sequels to Half-Life 2 and continue the story from where the main game left off.

Playing them in this order will give you a comprehensive understanding of the Half-Life universe
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