Too bad for Tenchu. Fun game, hands down the best ninja game ever made. But Half Life, well, it's just flat out amazing.
Although the sequel is definitely superior in several ways, the groundwork was laid with the original of course. A deeply engaging story, and one of the best FPS ever made, as well as one of the best Sci-Fi games ever made. Great physics, graphics, everything. Top notch all the way, and deserving of everyone's vote.