I think lost planet colonies is up there in demanding....
try have it on max settings then add the directx 10 settings and then add 1680x1050 resolution and you're barely kicking in at 25-30 FPS on the good parts......
I also think GRID is a pretty demanding game...( try spining out and having the smoke particles cover the screen on max settings you see some slow down)
crysis still takes the number one spot on most demanding...I can play it with ok FPS...like 30 on very high...but on ultra high it's like 15-20.....
I haven't really ran into any other games where I see a big dip in FPS all other games run perfectly on max...