The oldest major publisher of science fiction is probably Ace Books, and if you enjoy reading older science fiction, as I do, you've probably read dozens of novels that they published. Ace published the early work of Robert Silverberg, Ursula K. Le Guin, Philip K. Dick, Samuel R. Delany, Roger Zelazny, and dozens of other authors who are now counted among the masters of genre fiction. Ace gave many authors their first exposure to a wide audience via their "Ace Doubles", which included two novels in one cover, allowing them to feature up-and-coming authors in a pairing with a more established author. However, in the 1960s, Ace published unauthorized copies of J.R.R. Tolkien's Lord of the Rings books, essentially trying to sell books without compensating the author. For this, and other lesser sins, I simply cannot name Ace as my favorite genre novel publisher.
Another possible choice was DAW Books, which was the first dedicated science fiction and fantasy publisher, and which has published novels by many of my favorite authors. DAW was founded in 1971 by science fiction editor David A. Wollheim after he left Ace Books, and has published more than 1,500 books since then, including many books that I love, including some by C.J. Cherryh, Andre Norton, and Fritz Leiber. In a very real sense, DAW Books was the only competitor I considered for Tor, and on a different day I might have picked DAW instead of Tor when asked this question. A distant third place would go to Baen Books, but only because they simply haven't had the highlights that DAW and Tor have had: Any imprint that has published Catherine Asaro, Larry Niven, and Lois McMaster Bujold is a first rate publisher.
But in the end, Tor gets the nod. Tor consistently produces the highest quality science fiction and fantasy content, which should really be that much of a surprise because the list of editors who have worked for the company reads like a "Who's Who" of genre fiction editing. Tor has been honored multiple times as the best science fiction publisher by the Locus reader's poll, and books published by Tor have won or been nominated for more genre awards than books published by any other publisher. In short, if you pick up a book published by Tor, you're likely to be holding a good piece of genre fiction in your hands. If that weren't enough, Tor has established a policy of having exclusively DRM free e-books, which moves them up in my estimation by several notches.
Ace, DAW, Baen, and even Ballantine. I'm a fan of all of them as publishers for the great science fiction and fantasy they have brought to the market over the years. But sitting at the top of the heap is Tor.
Go to Day 27: What Is the Most Epic Scene Ever in a Genre Novel?