Prettiest game is Assassin's Creed. The scenery was the star of this game and climbing some of those buildings and looking around was incredible.
Biggest letdown was Mass Effect. This game just didn't hold my attention, I quit playing it after about 15 hours or so. The storyline was excellent, I loved all the different races and galactic politics, and interaction with the crew but the meat of the game felt boring. The weapons/armor/powers just weren't that interesting and the environments felt hollow and fake. Like if you fell on a rock it would collapse like on the set of an old movie.
Ending Song on Portal:http://www.youtube.com/watch?v=KjGfgV7rJHI