Without going in to how they stack up now, there were games that I played that pushed the boundary of what I thought was possible on the machine further. The first game I played extensively on the 360 was Gears of War, and I thought it was a great feat. Here are the games that momentarily had me in awe when I first played them:
Bioshock
Resident Evil 5
Mirror's Edge
Final Fantasy XIII
Crysis 2
Rage (in hindsight, it kinda looks a little washy)
Skyrim (not so much the pretty visuals themselves, but the scale of the world factored in)
And games that have cool art styles:
Prince of Persia
Borderlands
Limbo
Mega Man 9 and 10 (sure, it's simplistic, but these truly feel like the old Mega Man games)
Mirror's Edge
Halo 4 is also an appealing-looking game, and the CGI is awesome. It might just be the epitome of what a 360 exclusive game should be like in the visual department.