Whether you started with a 2600 and a joystick in your hand, an N64 with a blistered palm or building your first PC in your teens, what is that one video game you've played at some point that to this day sits at the top of your list.
In high school we hid copies of UT99 on the file share so we could pull it down to our crappy Dell / Netware computers and bang out a couple rounds before class. It was the best.