Half Life 2.
Valve does a great job making a game feel right. The movement in HL2 works really well and the shooting mechanics are all solid.
They are also very good at establishing the world: you can see stuff just beyond the playing area, the soft loading mechanism that minimizes the break in action, and adding NPC interactivity at key spots to give you the sense of a living world.
Now, you have to realize that HL2 is a game that pushed the genre forward tremendously. Before HL2, worlds were just a series of loosely related maps, connected by predictable doors. NPCs were minimal and served mostly as set dressing. Play a FPS game before 2004 and then notice how much they changed afterwards. You HAVE to appreciate that.
As far as the game holding up, well I think it does. It still looks attractive. It isn't as detailed as a newly released game, but the style and artwork stand the test of time. It plays well. And most importantly, it tells a good story. YES it gets bogged down at the watercraft portion of the game, but if you push through that you're going to have a blast. There are some truly great set-pieces to be experienced. The follow up Episodes round out the story nicely.
There aren't many FPS games that pushed the genre forward as Valve did with HL2, and there certainly aren't as many that hold up as well. It's still an extremely competent shooter, and it deserves respect for what it did and what it is.