Science fiction has been my core reading fodder. One way to tell your true literary, or musical, or culinary, home is to look for the places where even mediocre works give you pleasure. In literature, that's SF for me. I like the "what-if" thinking, I like the consideration of alternate social structure, either explicitly for humans or as reflected onto alien societies, and I like the generally forward-going outlook.
Oh, and as with the Hugo and Nebula awards, and eligibility for SFWA membership, fantasy counts as science fiction.
I also like the sorts of characters many SF books contain: competent people who actually do things. Sometimes they win, sometimes they lose, sometimes they make mistakes, sometimes they get hurt, but they aren't wanny-woos. Wanny-woos are boring.
I can never get these down to a small enough list, but these are my really truly formative books and authors.
The history of science fiction is remarkably well documented. It's short, and many of the important people involved in it were writers, so it's often interesting to read.