What do you consider to be your "formative game"?

For as long as you've been gaming, which title from your past can you point to and say "this game is what made me love this specific [genre]/[play style]/[art style]/[etc]"?

I'm sure that everyone can point to more than one, especially as we get older and experience different games that interest us in different "silos", but if you had to pick just one, which one would it be?