I would say Crysis or Metro.
Crysis has all that dynamic vegetation (that no other game has really had as much of).
If you mess around with the CryEngine2 editor, you can really get a feel for how much it (vegetation) kills FPS. You can build an environment typical of something like COD or BF (mostly static buildings, low vegetation density, etc) and get really high FPS - - but you start throwing in a lot of beautiful waving plant-life - give it dynamic shadows, high-res textures, long draw distances, etc, and you'll bring everything to a screeching halt.
Everybody thinks CryEngine2 is so terribly "unoptimized," but it certainly doesn't seem like that to me - - you have to consider the kind of environment that's being rendered. The Metro engine doesn't have any of this large-scale dynamic stuff to deal with and it still seems to really pull things down.