OK, I voted Deus Ex, because this should take you less time (ass opposed to Dark Souls and Oblivion, for example) and free you up to play more stuff, but I missed the Dishonored entry. I'd vote for that. I think it's shorter than Deus Ex (well, haven't played DE, yet, though). For the record, I have 44 hours on Dishonored, which is two playthroughs (first one wasn't completely thorough, the second one even less so) but you can extend the playtime, if you disable the rune and bonecharms pointers (you can still find them all using the heart, though). I would have loved to do that, but I was in a hurry to finish it quicker and start another game.