Half-Life 2 is, in many ways, the perfect game.
The best game I've ever played? I would be hesitant to award it that honour. I've played GoldenEye 007, The Legend Of Zelda: Ocarina Of Time, the original Half-Life, Perfect Dark, Metroid Prime, Ikaruga, Deus Ex and Super Monkey Ball. In terms of total entertainment provided, no. I ran through HL2 too quickly (~18 hours) and it has too little replay value. Ikaruga, a five-level scrolling shooter, beats it out, let alone Perfect Dark, a game I spent a horrifying 400 hours playing and re-playing.
But in other areas, HL2 excels, blisters, and blows minds.
...HL2 is astounding. In-game graphics, run at high detail level (I have an Athlon 3200+/Radeon X700/1GB) are solid, intricate and very, very convincing. The level of detail is comparable with that in Metroid Prime, mentioned above, placing it equal first in my experience. (I've not played Far Cry, which is said to be better yet, but whatever.)
Let me digress a bit on photorealism. Photorealism is seen by many as a (currently unobtainable) Holy Grail in gaming. Next-generation FPSs and racing games proudly show the astounding, indistinguishable-from-the-real-thing images they've managed to come up with in labs - I'm sure we all remember the Alfred Molina/PS3 demo at E3 2005. Amazing, yes. Good for gaming? NO. The major point of videogames is that they enable us to do things which we cannot do in reality. The real world is BORING. As such, strictly emulating reality is a bad idea, because when (inevitably) something happens in the game which couldn't happen in real life, suspension of disbelief is shattered in the most jarring way. Not only that, but setting the bar high in terms of graphical splendour simply pushes smaller software houses out of the game, and forces the big ones to spend hideous amounts of time and money on games which we'd rather prefer arrived sooner, cheaper, and longer.
HL2 falls short of photorealism by precisely the right amount necessary to accomodate the obviously necessary shortcomings in realism (health is a number out of 100, the player can carry a half-dozen heavy guns plus ammo, etc.). The world of this game, a regular (eastern European?) city, is breathtakingly real, and the character designs are really, really, really good. Not that-many-reallys convincing - they aren't photorealistic - but they look and move like real people. Their skin looks like skin. Their faces and fingers move like real faces and fingers. The animation is breathtaking. A face capable of displaying realistic emotion in a videogame? Perhaps I've been out of it for a while, but this is sensational stuff. But while they aren't 100% real, the characters fit the environment. It looks like it was all drawn with the same paintbrush, which is very much a more important factor in videogame graphics these days.
Audio is never something I've been hugely picky about. Sound effects are sound effects, after all, and passable music (as found here) is not, I would say, difficult to come up with (but what do I know?). More pertinent is the question of voice acting. The voice acting here is superior, but there's some proper emotion going on in this game, and I find myself slightly less convinced during some of the more emotional exchanges. Alyx, for example, is less convincing than Breen, who is acted with passion and finesse. Gordon, of course, is played better than anybody. :)
If you've played Half-Life, what you can expect is very much a similar experience, but refined and polished and supercharged. There are puzzles, well-defined areas of the game which you need to stop and think about for a bit until you can get to the next bit. There are set pieces which are invariably extremely well-scripted and fit slickly into the game world. New weapons and abilities are introduced just as you start to get comfortable with the existing formula. Combat is highly enjoyable, enemies are highly intelligent and behave more like real enemies than any I've seen before. As a one-player experience to cap all others? It's got stiff competition (above), but it's immersive and compelling and utterly brilliant and definitely up there with the best of the very best.
I should probably mention the Havok physics engine employed to superb effect in the game, and now I have.
Too short? Too long? Well, I'd have enjoyed a game which was twice as long, but it wasn't too short. Nor was it too long. The plot? The basic events depicted in the game were transparent enough, and well-exposed, but no questions remaining at the end of the last game were answered in this one. I'm told Half-Life 3 is planned, though, and I'd like to think that sooner or later the whole truth will be revealed.
A milestone. A shining star. The beginning and the end of an entire era in videogaming. Half-Life 2 raises the bar for other games by an unimaginable amount and, I would say, demonstrates what is basically the best game possible within our current perceptions of adventure games, shooters, and a half-dozen other genres. The question many people will be asking now, much like they did in 1998 with the original Half-Life, is "How can any game match this?", but the question I ask is how, even theoretically, can Half-Life 2 be surpassed? Where do we go from here? What is the videogame industry's answer?