For me it's Chris Sawyer's Transport Tycoon (Deluxe), without a doubt. That game was and to this day is (in the form of OpenTTD) like a highly addictive drug fo

For me it's Chris Sawyer's Transport Tycoon (Deluxe), without a doubt. That game was and to this day is (in the form of OpenTTD) like a highly addictive drug for me. If I start playing, I'm compelled to optimise my transport network for many hours. I got it when I was 11, and incidentally, was the first computer game I bought with my own money. Oddly enough, when I introduced my girlfriend to the game, she instantly showed the same obsession with it. At university, when most other people were out drinking, we'd occasionally start playing Friday evening and play for most of the weekend. Although I didn't start programming based on it (I started programming because soldering increasing networks of an increasing number of logic gates became tedious and expensive) I did start learning graphics programming because of it. I dream of creating a worthy successor to it one day. (I'm too scared to start, considering what playing it does to me)