I think they call this the "gateway game". The term is derived from the idea that there are "gateway drugs". Drugs that aren't too dangerous or addictive but that get someone used to the idea of "using" and eventually can lead them to harder more addicting drugs.

My gateway game would've a pong varient called the Magnavox Odyssey 300. I LOVED the hockey game and could beat anyone in my extended family at it when I was about 7 or 8 years old. Uncles, Aunts and Grandmas all fell to my terrible Hockey Pong onslaught.

My first "killer app" the one too hook me for life was Adventure on the VCS.

If I had to start accumulating games all over, that's the first game I'd seek to get back. Which is exactly what I did when I restarted this amassing compulsive disorder back in 2002.