It's interesting that we all seem to go for stylized aesthetics when asked for preferences. Though I think some of the more realistic-looking games are very impressive aesthetically as well.
I've always admired Tomb Raider 1's atmosphere and colour palette. And I feel the developers were never able to match the first game, aesthetically, no matter how much technology they threw at it.
Few people think of Grand Theft Auto 3 when talking about game aesthetics. Yet I think it's a very beautiful game, again mostly because of its subtle colour palette.
Takayoshi Sato's character design in Silent Hill 2 is unparalleled!
And then there's of course Ico, the aesthetic masterpiece of the Playstation 2.
Guild Wars' vistas can be quite stunning. And its characters are the most elegant I've seen in a multiplayer game.
When Half Life 2 came out, people seemed to be attributing its beauty to the Source engine. But it as really the work of the art directors that made the difference!
I think Kane & Lynch 2 is also looks very interesting, with all its simulated video artefacts.
And I found the cities in Assassin's Creed (both 1 & 2) very impressive, aesthetically.
(though I wish we could redo their ambient sound design...
).