The original Half Life is pretty fun, but I’d recommend checking out Black Mesa. It’s a remake of the original game in the same engine that Half-Life 2 uses, and changes some things for the better.
Gotta be Portal 2. The puzzles are fun, the difficultly curve is reasonable, and the writing is fantastic. Truly the only complaint I have is that I wish there was more of it.
The original Half Life is pretty fun, but I’d recommend checking out Black Mesa. It’s a remake of the original game in the same engine that Half-Life 2 uses, and changes some things for the better.