By today's standards, Half-Life and Half-Life 2 are average. That's pretty nifty for things that came out in '98 and '04 respectively. And I'll warrant that even modern games fall short of what they did.
Remember the tentacle monster? That was one of the first times a game ever asked NOT to take on a boss head-on. The initial Black Mesa outbreak which left you weaponless for a few minutes was one of the first times a shooter afforded me a bit of genuine tension. I don't know if you remember; but most shooters in those days had one method of dealing with enemies: shoot them until they fall over.
Running, screaming, and crying as the Gargantua chased you down is still something few shooters today have the balls to do.
Then there was Half-Life 2: HOLY SHIT! That was the first time most of us have seen a physics engine, and what an opener it was! Yes, using floating barrels to raise a bridge may seem passe now, but then it required us to think about our videogames in a way with which we were very much NOT accustomed.
Then, of course, there was the variety of gameplay and enemies. For the first few levels, they've got you running across rooftops and trading pistol fire like you're Jason Borne. Then you're dodging mines and machineguns in an unarmed airboat. Next you're listening for the sinister chirp of a Black Headcrab or the howl of a fast Zombie in Ravenholm. But it's not over yet! You've got to play 'the floor is lava' with the antlions (until they actually give you the opportunity to COMMAND the antlions), scramble for rockets as you fight gunships (since the fuckers keep shooting them down), face down a squadron of Striders, and finally: complete the entire end of the game with nothing but the gravity gun.
Wait, wait, wait...I'm sorry. What I said about it being average...I take it back. Half-Life 2 STILL puts 90% of the shooter market to SHAME. I'm wracking my brain trying to remember a single other game that let me do so much in one experience. Even the Elder Scrolls, with their open worlds and 1,001 approaches to gameplay, don't offer so much in one package.
Now I'll be the first to admit that I've put my nostalgia goggles on, but I'll be damned if any other game made me as tense as I was when I was low on ammo and listening to the ever-nearing sing of manhack blades whistling through the air.
Half-Life 2 is an excellent game (in my opinion); something I would definitely submit as the unqualified best game of all time. Yeah, it's got its problems: the guns are a little plink-plinky. The story itself is kind of...meh. But when that airboat sailed through the air as I ramped off a concrete pipe, helo impotently firing just to my left; I felt like an honest-to-God action hero, and no one has been able to equal that experience since.