For me, the most important one was Diablo 2 because I spent way too much time in it. I got totally pulled into the addictive/gambling side of farming and trading gear, and, there were definitely some opportunities in life I missed because I was spending way too much energy on this.
I know this was the opposite thing you were looking for in your question but what this taught me is how exactly it feels when I get myself caught in this trap. I’m much better now at seeing myself falling into that mindset and then making changes, which has made a world of difference.