I don't need to tell anyone who follows this site how important music is to a movie. You guys know. And I know I don't need to tell you how important James Horner was to movies of the last few decades. The man's IMDB page reads like a list of movies we've all seen in the theater since we were kids.
Tragically, Horner seems to have died in a plane crash.
Rather than talk about who James Horner was and what he worked on, I'm going to invite you to drop your favorite of his works in the comments. If you want to add a why and wherefore, feel free to do so.
I will say the first score of his that I remember consciously really paying attention to and wondering "who did that?" was the score for the 1989 Civil War film, Glory. Certainly the score wasn't the only thing Glory had going for it. but it set a new standard for the music in historical epics and I often feel like both documentaries and more recent movies have lifted from that score, or at least learned its lessons.
and, of course, I'm a fan of many more of his works.