They’re not at the top of my list, but Burroughs’ Martian series and Doc Smith’s Lensmen series are giants of the genre. Theirs were the shoulders that Heinlein, Asimov and Clarke stood upon.
I like Pournelle’s Falkenberg series; not so much science fiction, but well-written narratives of the great military campaigns of history, transposed into the future.
Niven and Pournelle teamed up to write some blockbusters. Lucifer’s Hammer is a great “end of the world” survivalist story. The Mote in God’s Eye and its sequel The Gripping Hand are great “first contact with aliens” stories.
H. Beam Piper’s “Fuzzy” stories are sentimental favorites of mine.