I'm yet to find a FPS as engaging and as nice visually as HL2.
Every other FPS of the next gen techonology has numerous flaw that you won't find in HL2. While every new FPS coming out now use Normal Mapping and such, only HL2 manage to use it correctly. Remember how everything in DOOM3 look like metal or plastic? Well, check out Resistance of Men on PS3... The same problem happen once again. So far, only HL2 manage to make sand look like sand and avoid have wood look like plastic.
But the visual is one thing. While not being totally innovative on the shot them up type of game, HL2 is in my opinion the first real FPS to use physic as a gameplay ingredient. Sadly, they just didn't use it enough. But the last level of the game worth the game on itself as you grab your grav gun and make the combines dance!
As for the story? Like the first Half-Life, the story is a wicked crazy thing that even after passing the game 2-3 times, you're not sure to understand everything. And even then, so many thing remain hidden in this complexe story. Of course, the perfect dumbass can pass the game without noticing any story at all. But the amazing thing is, someone who stop and take care to listen to every and read between lines can see so much more. Who is really this agent with a suitcase? Aside from the really nice gameplay, tremendous physic and stunning visual, this game also show one thing... When the developpers take their time, it worths it.
Yes, even today, a year after this game came out, it's in my opinion, still the best visually speaking.