Of all time? I'm a sucker for Ennio Morricone too.
I like Phillip Glass' stuff, and I like Yann Tiersen (even though he has only scored a few films). I'm also a huge fan of Danny Elfman.
I do love John Williams, but yeah, his stuff has a certain bland quality to it, it's hard to explain. I mean, I wet my pants with glee whenever the Star Wars theme is played loudly, I can't fucking help it, but I don't have a lot of his stuff on my MP3 player. His stuff is like comforting junk food to me. I don't mean that in a snarky way, I just mean that it has more of a pop music feel to me.
"Nobody in life gets exactly what they thought they were going to get, but if you work really hard, and you're kind, amazing things will happen. I'm telling you...amazing things will happen" --Conan O'Brien
He is the composer all other composers aspire to be. His music transcended the genre and for the first time motion picture scores became truly 'pop'. Guys like Silvestri, Horner, Zimmer, Giacchino: you wouldn't even know their names if not for John Williams.
I usually hate soundtracks and I find most of them boring and uninteresting. Most of all traditional scores with a standard orchestra and cheesy melodies. (Don't want to be mean, I don't name any composers to not offend anybody)
But I enjoy Hans Zimmer's works very much. His music gives me thrills every time I listen to it, just great. I don't like the older ones that much (<2000s).
Favorites: The Dark Knight Triology (Co-Composer: James N. Howard), Gladiator, the Kung Fu Panda movies (together with John Powell), the 2 Sherlock Holmes, the Pirates of the Caribbean series (I think he has done the first one together with Klaus Badelt), Inception, Angels & Demons... I think that was it.
Oh, and I love John Powell's score to How to train you Dragon. Really one of the best musical pieces of all time.