It's definitely Deus Ex. There are other games i have played that had such a memorable aesthetic or emotional impact that they stuck with me years later (Mirror's Edge, NORTH and more), but Deus Ex is a game that i keep going back to play again and again. It just does so many things right.
Deus Ex came out pre-9/11, but it predicted the war on terror and how it would be used to expand the surveillance state and militarize the police force. You play a counter-terrorism agent who starts questioning the value of the oligarchy he is employed to protect. Being a cyberpunk game, it also turns the conspiracy up to 11, so every wacky shit from Robert Anton Wilson to the X-Files is part of the neon-drenched soup. The protagonist is doggedly earnest, and the exchange about why he wears sunglasses at night remains one of the most hilarious in any media.
Subsequent games got a new protagonist who was a more traditional "tough guy" and were far less interesting for it.
Deus Ex is frequently mentioned in "Greatest Of All Time" lists, so i suppose this isn't a very interesting choice. On Kotaku UK you can read you can read someone else pontificating about the presience of the game: https://www.kotaku.co.uk/2014/08/05/deus-ex-predicted-future
2. What is your earliest memory?
Being on the beach in Folkestone, while my father and his army colleagues piled up sandbags to defend against a storm. It must have been the early 1980s, around the time of the Falklands War.
3. If you could have one wish fulfilled, what would it be?
Freedom of movement for all people, anywhere in the world.
4. Have you ever lost something that is important to you? Were you able to find it?
I am sure i have lost things that seemed important to me at the time, but apparently none were important enough that i remember them now.
5. Would you rather go scuba diving or rock climbing? Why?
Rock climbing. I am not particularly interested in hanging off the side of a mountain, but i think i'd feel more comfortable in an environment where i can see for miles and don't have a gauge counting down the minutes of life i have left.