Wow, I can’t believe I just found this thread and not one mention of Jack Vance. His "Planet of Adventure" series, Alastor series or Demon Princes novels are classics. His command of language and dialogue is unparalleled. Every modern sci-fi author cites him as an influence.

Gateway by Frederik Pohl is another underrated one from the 70s although the sequels are disappointing.

The Forever War Joe Haldeman and Robert Heinlein’s Starship Troopers are also indispensable.