I'd definitely have to say John Williams. He created so many sci-fi/fantasy scores that have become, well, timeless classics, I guess.
In addition, I think there's a great variety in the material he produced. "Star Wars"*, for example, is a completely different beast compared to the amazing "Close Encounters of the Third Kind" or "A.I.", for example.
There are definitely many amazing composers out there who've created sci-fi/fantasy scores I love (e.g. Vanglis' music for "Blade Runner", Goldsmith's "Total Recall", Poledouris' "Starship Troopers", and just many, many more). But I think nobody comes close to the body of work that Williams managed to create over several decades.
I definitely also have to mention Bear McCreary. He's still very young, and it's still pretty early in his career. But he has amazed me with the work I've heard so far on nuBSG. If he keeps up like this he could well take over the number one spot. But there's still a long way to go, I think
*I'd just like to briefly comment on the "Lord of the Rings" music. Personally, I was and am still very disappointed by it, actually. I don't perceive the breadth and depth that was mentioned by Neroon, to be honest. Instead, I find it to be, well, rather lacklustre in its use of 'epic saga clichés' such as the choir that always kicks in when things are supposed to be particularly impressive.
The reason I mention this in connection with the Star Wars scores is that I remember hoping that the "The Lord of the Rings" music would be like a modern version of the "Star Wars" scores, in a sense, delivering the same musical variety in particular with respect to the characters and their themes. But what I heard in the end I found to be far less exciting, varied and engaging.