Trek III is the film that expanded the universe the most, and it has the grandest scope. So many new ships. So many new characters. Movie Klingons for the first time. It's also the most daring. Killing David, destroying the Enterprise, turning the crew into renegades. And James Horner's soundtrack! The soundtrack!
It also successfully merges the TMP style with the TOS style.
It's the best one, together with TWOK.