Since reading Mission of Gravity, I've gone through everything of Hal Clement's that I can find.
A year later, I can say that Mission of Gravity is definitely his best.

It has a few scientific problems in the harsh light of the 21st century, but overall it's fabulously well-thought-out hard science fiction.
Mission of Gravity features a limited cast of characters, a slow but steady progression of trials and setbacks and a satisfyingly optimistic conclusion.