Some great mentions here, Williams of course, Zimmer, and Horner (I used to keep the Willow soundtrack on repeat while studying during college).
However, Williams is easily my favorite. As much as I love the The Empire Strikes Back end theme and Raiders of the Lost Ark main theme, I have to go with Superman: The Movie main theme.
And funny story behind that, back during college I was at Best Buy picking up the soundtrack on CD. I get to the cashier - who's really cute - and not wanting to reveal how much of a nerd I was (because it wasn't cool yet to be a nerd back then lol), I turned it around while having my hand covering the title and held it out so that she can easily scan it and save myself from embarrassment. Well, she snatches it out of my hand, flips it around, and then looks at me and tilts her head down a bit and says, "Suuuuuuperman?!!" She at least laughed it off though after I made a some wise-ass remark.
Same thing also happened with the Grease soundtrack, which I will add that entire soundtrack gets an honorable mention.