It's Half-Life. An important piece of gaming history that revolutionized a genre. You owe it to yourself to play it if you haven't already. It shows its age in some ways (apart from the obvious, like graphics and sound), mainly because it's been copied endlessly. But its still a blast, and it's still easy to see why it was so highly acclaimed in 1998.
It was Valve's first game though, and even though they struck gold with Half-Life, their inexperience at the time shows through on a couple of chapters. Notably "On A Rail", and of course, most of the Xen chapters. This isn't their magnum opus. No, that came in the form of the sequel to this game.