This is kind of an odd thread, in that it would seem that being hard to run would imply the game being graphically superior in some way, when it obviously isn't the case.
I think the real issue here would be what game actually looks the best that is hardest to run(?)
Any modern game engine is capable of bringing the system down....you just start increasing graphic variables until the box can't go on.
I like to mess around in the CryEngine2 editor, and it is pretty easy to drag things down in a hurry....just start giving everything shadows (even the grass), increase view distances, etc.....just look at the shot below of a big map I've been messing with:
If you look in the display info you'll see I'm only getting 15 fps with my 570 GTX, and that is
without being in-game with any AA, or even a high resolution. Start adding AI into the mix and things really drop.
http://earthmachine.home.mchsi.com/crysisfps.jpg
So I think the bigger issue here may be which game, or engine rather, can be pushed the farthest while still delivering playable frames.