Prettiest game world would have to be Oblivion, but it's boring visual effects, substandard character textures and poor animations leave the overall game wanting.
VtM:Bloodlines is a visually stunning game with great gameplay that suffers from activision's unrealistic min and rec specs. The game does not look or perform well even on a rig meeting the rec specs. Throw enough machine at it though, and it may still be the best looking SOURCE game made, at least until HL2: Episode 2 is released.
NWN2's official campaign stinks, but the visual effects in this game are stunning.
Jade Empire SE for the PC. While the graphics themselves may not be the best out there, the better textures help a lot though, I just love the way this game looks. The character animations are superb in my opinion, probably because they were motion captured. The visual effect are a bit substandard though, being not much better than Oblivion's.