DOOM II: Hell on Earth was the first game I played on PC back in 1995, albeit in shareware form. It came on a 3.5" floppy disk by the means of a friend of a friend of my father's.
Being only 10 years old at that time, I wasn't allotted many gaming hours in my daily routine, so I had to make them count — and what better way to do so if not playing the most gorgeous, violent, realistic-looking game ever made? I remember I spent Sunday mornings smelling the scent of lunch being prepared and killing Imps, running away from Cacodæmons and swearing when facing Hell knights. I was genuinely scared, everything looked so damn real and engaging, and my heart was racing the entire time — I had so much fun, even just by remembering all of this!
You should play DOOM and DOOM II immediately. Like, right now.