I think Gears2 (or at least Gears1) uses many tricks to look as good as it does. If you look closely other games and Killzone2 have far more details and many of the great Gears2 effects are for example background images.
I mean it still looks amazing, good tricks are also important
But if you look at the screenshots:
http://uk.xbox360.ign.com/dor/objects/14232680/gears-of-war-2/images/gears-of-war-2-20080714001316563.html?page=mediaFull
It looks amazing but not much of that is actually 3D graphic
http://uk.xbox360.ign.com/dor/objects/14232680/gears-of-war-2/images/gears-of-war-2-20080714001320188.html?page=mediaFull
Characters look pretty cool but its hard to make out little details, everything looks so flowing together. Perhaps some higher textures instead of more shader effects would make that easier.
http://uk.xbox360.ign.com/dor/objects/14232680/gears-of-war-2/images/gears-of-war-2-20080707034713224.html?page=mediaFull
And if you look at the actual 3D structures it looks very nice but not that outstanding.
I am a friend of clear, sharp images so I would give the nod to Killzone2 but Gears2 really looks like a render movies at times so its also a worthy contender.







