Isaac Asimov is great. I reread the Foundation Trilogy every year or so. But I can't say he's my favorite.
My favorite writers are (in order): Arthur C. Clarke, Douglas Adams and Philip K. Dick. The first book I truly enjoyed -- and actually read for recreation -- was 2001: A Space Odyssey. I read all of Clarke's books that year and was probably around 14 years old.
He definitely opened my eyes to reading. Ever since, I've been addicted. For that reason alone -- and because I thoroughly enjoy his books and writing style -- he is my favorite.
Now, for me, the hard question is, do I like the Rama series better than the Space Odyssey series?