This was a bit harder to decide upon, but DE:HR seems like the right choice. This was the first Deus Ex game for me, because obviously I was in the thrall of EverQuest during the release of the original Deus Ex game and never had a chance to play it.
I love the world of Deus Ex and especially Human Revolution. I've gone back to play the original and found it to be good, but it is sometimes hard to go back in time when it comes to games. The downgrade in ease of use, UI and graphics were too much for me to enjoy it as much. But more than that, I loved some of the design choices made in DE:HR - I really want to live in Jensen's apartment.
The main point, however, is that on this list I am valuing games that affected me in various ways more, than those that just had good gameplay or story. I played Deus Ex: Human Revolution when I was considering a career change. I had been thinking about going back to study something and doing something different, but I could not settle on any one thing. I wasn't able to figure out what I wanted to do. Then I played this game and was sucked into a wonderful scifi world (ok, I guess it's a dystopia, but still) with all sorts of technological marvels. But most importantly, they were technological marvels that seemed achievable - they weren't the super advanced nanomachines of the original game, but rather robotic prosthetics that could actually be built someday.
I had thought about a career in engineering, but I was always apprehensive about the amount of math and physics I would have to study, so I never really considered it a real option. Until this game. It ignited my passion for...I don't know, future technology? Science? The end result was that I started looking into engineering degrees, picked one that interested me (biotechnology) and applied for it. I discovered that I had underestimated myself and had been apprehensive for no reason. These days I've already graduated and have been thinking about advancing my studies even further.
Perhaps I could've reached this point eventually on my own, but I have no idea how long it would've taken. This game, aside from being a really fun immersive sim, was integral in creating the person that I am today.