[w-ll]

HL2 was a flagship for a newer game engine. The physics and details were amazing when it came out, and then you had other games and mods like Garys Mod that was just an amazing sandbox of the engine in the game.

[akhilcacharya]

Character models remained best in class for several years after.

[bhhaskin]

Don't forget about the fantastic AI. Being able to dynamically ract to sound and change tactics was pretty cutting edge.