My first thought was as well Beyond Rangoon ... although, like someone already said, it's not really underrated, but more not so well-known. Although it's obviously more well-known now it seems. Only a few years back, most of whom I mentioned this too, didn't even know about it. But now it seems to have changed.
Still hoping to get the complete score for this one day
Underrated scores from him ... some of these perhaps:
Broken Arrow The Road To El Dorado Spirit: Stallion Of The Cimarron Pirates Of The Caribbean: At World's End
Well, I'm not particularly a Zimmer fan so maybe I'm not allowed, but I'm going to go ahead and nominate his lovely score to Spanglish, a wonderful film which his Hispanic-flavored score complements perfectly. If most of Zimmer's output was in this vein I *would* be a fan! Too bad he and his gang have taken over almost all the big tentpole pics because I think where he excels best is in smaller, more intimate fare...
I think by its qualities The Prince of Egypt is his most underated score. Just listen to it. Not only doesn't sound like a typical animation score but one of the main themes (the one that illustrate Moses and Ramses brotherly relationship) sound like something inspired on the holocaust. Just beautiful and sad.
KING ARTHUR makes a push, but GREEN CARD gets my vote. It's funny that Zimmer has become synonymous with the wall of sound approach for action movies, when his comedy work has always had such a light touch. GREEN CARD perfectly juggles comedy and drama and the result I think is one of Zimmer's very best (and most attractive) scores.
I can't believe no one has mentioned Thelma and Louise yet. That score so perfectly fits the movie. I love the guitar in that score. I'm so glad we were able to get an official release from Kritzerland.