Team Fortress 2 is an indescribable beauty on every level and is placed within my top 3 games of all time.
When the main menu loaded my opinion was instantly turned. You are greeted by a still picture in the background, and an oddly amazing theme music. That in itself sets the atmosphere beyond words already, even with steam's static menu interface. I loaded up a game and was greeted by a humorous video explaining the map and how it should be played. I continued to play and was impressed by the diversity of the classes. Over time I got a little more into it until I finally listened to the developer commentary included with the game and my view about TF2's graphics changed forever.
After you hear about the art design, every single little aspect; every light, every texture, every pipe means something. It goes from being a cartoon to being a work of art to rival some of the greatest works of the century. They didn't created an entirely new shader model just to make system requirements lower, it was created to perpetuate the inescapable and perfect atmosphere of the game.
The gameplay is outstanding. After over 100 hours on the game I have yet to get bored. I got bored of Call of Duty 4 in about 40 hours. Every class has almost limitless re playability due to the number of maps, the dynamics of every class, and how they are used depending on what the other members of your team are. Sure, there are times when your team sucks, and that can ruin the experience, but that is the case on any game (unless you are playing mindless DM action). But even in these cases the game can still be enjoyed. I have sometimes had more fun sneaking behind the far superior enemy team as a spy and wreaking havoc on the unsuspecting dominators of my team than working flawlessly with an excellent team. The experience with the other players is ultimately what you make of it, and beyond that, TF2 is artistic and gameplay perfection. I have never found a game before this one that I could find nothing to mod on.
Team Fortress 2 is just another example of the limitless beauty that can be derived from the Source engine.