There are few games as well designed as Half-Life. Not only there's a nice selection of weapons, both conventional and experimental, and hordes of horrific enemies, but it's all wrapped in a very interesting setting. However, the greatest strength of the game for me is the pacing; there isn't a dull moment in the game unless you get stuck and there's a surprising amount of variety for the main campaign.
The only downside to me are the vehicle segments, which I've never enjoyed, and that enemies are too spongy in the highest difficulty.