If you are a genuine fan of science-fiction and fantasy you have to have at least heard of Tor Books. The imprint of Tom Doherty Associates is one of the most reputable in both genres. The Wheel of Time, John Cleaver, and Enderverse books were all published by Tor.
Somewhere around twenty percent of the books on my shelves were published by Tor, to my estimate. I haven't read all of them yet, but certainly will. Brandon Sanderson, one of my greatest mentors, even if he has no clue who I am, was published first by Tor Books.
I haven't found a book by Tor that I didn't like. The Wheel of Time series is incredible. Occasionally I find something that could have been changed to make it better, but for the most part it gets a 10/10. Ender's Game was pretty revolutionary for me. I hadn't read much sci-fi before it, so it was definitely a good intro to the genre. Speaker for the Dead and Xenocide showed me that not just literary fiction can delve into philosophy and psychology large-scale.
Please go out and support Tor Books. I just recently picked up David Weber's Out of the Dark.