hmm, do you mean artistically or technically?? some games aren't very attractive because the setting dictates that its not supposed to be very attractive, but they still have the technical features that makes them look fantastic graphically ("STALKER" and "Gears of War" are excellent examples).
as far as the game with the best graphics, Oblivion definitely ranks very highly on my list, but after playing Half Life 2: Lost Coast, i know that HL2: Episode 2 has potential to look much better than Oblivion (and that's saying quite a bit). overall, though, Gears of War completely blew me away with how good it looks when i played it. so, it gets my vote.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind