Transformers: Fall of Cybertron on Hard w/ both Prime & Megs.
I had played and beaten the game before when I was testing it at Microsoft, but I was finally able to buy the game (on Black Friday) for cheap & beat it on my own console.
I read it generally takes about 60-80 hours to beat the game, but it took me 26 hours. I think it would take longer if I did everything, but I'm not a trophy/achievement nut. One thing I cannot do is backtrack and collect things and search. A reasonable amount's fine, but after a while, I'm done.