Modded Oblivion hands down
My friend has a gtx 470, 6 gigs of ddr3, an i7920 oc at 3.8ghz and Oblivion slowed to a crawl with "qarls texture pack 3", "More than everything viewable when distant", and "4096x4096 LOD textures" installed. Went to about 8 fps.
Most people can play crysis, very very few can play oblivion with all the best texture mods. hehe