Unbeatable
But firstly the storytelling; it's revolutionary for a FPS. The story and twists and the levels are so creative. As you progress you feel like it's true. Unlike other games which skip some story telling scenes this game doesnt. It flows as if it's in real time. The Storytelling is so complex that a walkthrough is recommended.
The graphis of the game when it was released in 2004 is still very good even now when graphics had changed and evolved. When it was released the Source graphics are superb. A new benchmark was set by Valve to other developers.
The episodes released are also very good although "episode" is a good way to describe the length of them. But so as the Half Life series continues. I prefer and Half Life game to fully end the great Half Life series.