Agreed, Gladiator is my favorite soundtrack of all time, and I've never heard a soundtrack of Zimmer's that I didn't like. I'd put him right up there with John Williams even though he's only received a fraction of the awards.
I actually like Hans Zimmer a lot more than John Williams - the latter has a very distinctive style that can seem very in-your-face, more like an "Oh hey, this was done by John Williams" than something that best fits the film. Harry Potter, Star Wars, and Indiana Jones are all recognizable as Williams soundtracks, even though they're very different films.
Zimmer seems to vary his music a lot more to fit the needs of the film - it's just universally good, it's not universally Zimmer. The Lion King is very different from Crimson Tide, which is very different from Gladiator, which is very different from The Dark Knight and Inception.
BTW, I think Basil Poledouris (The Hunt for Red October, Wind) was another underrated film composer whose music is much better than his reputation.
> more like an "Oh hey, this was done by John Williams" than something that best fits the film
Really? I would be tempted to argue the opposite. The theme for Jaws, for example, is instantly recognizable and well known precisely because it fit the movie so well. Star Wars is near-impossible to separate from its music - from the main theme, to the imperial march - and his use of choral music in Phantom Menace had a big impact in the years that followed, much the way the "Inception sound" has been showing up all over the place in the last few years.
And citing the bigger action-adventure movies doesn't really make a good point. Williams has done a wide variety of music for movies, all of which is very good (Hook, Schindler's List, ET, etc.)
I do think music has changed over time in how it's used in film, and that Zimmer has a more "modern" style, where the soundtrack ties more closely to the emotional ups and downs than Williams does. (Nothing wrong with that, I like them both.)
http://www.youtube.com/watch?v=Z0kGAz6HYM8