Which Dune series? Frank Herbert or Brian Herbert?
"Ringworld" by Larry Niven series is good. Also the "Gateway" series by Frederick Pohl but it's kind of whiny. Those are hard science-fiction.
There are two books about the Moties, "The Mote in God's Eye" and "The Gripping Hand"--by Niven and Jerry Pournelle. I liked the first one better.
Robert A. Heinlein did a lot of powerful sociology science-fiction up to the 1970s. "The Moon is a Harsh Mistress" is about a penal colony that breaks free of Terran rule. "Stranger in a Strange Land" is a classic of the counterculture, a Man raised by Martians comes back to earth. "Starship Troopers" is a LOT better than the movies.
For an interesting twist read "Starship Troopers" and then "The Forever War" by Joe Haldeman. Haldeman lost a leg in Vietnam and his Galactic war story is about draftees who hate all authority. Try to get an older version, the "New Author's Version" is one where Haldeman tore up what Ben Bova taught him and shows his original draft of the story.
Keith Laumer does a series about a future Earth diplomat who has to stop Earth pansies from giving away our galaxy to alien dictators, usually by breaking into their HQ and slapping around their bosses. Its done for laughs. It's called the "Retief" series, the best are collections of short stories from the 1960s.
Maureen F. McHugh did a really great novel about a future USA hit with drought and overrun by Maoist revolutionaries called "China Mountain Zhang", a half-Chinese American learns how to deal while studying architecture on both sides of the Pacific, and side-stories about street kids hang-glider racing among NY skyscrapers, and their friends on a lunar forced-labor farm.
Ender was a good series but not my favorite. Hitchhiker's Guide was a great series but Adams died before he could make a happy ending in a sixth book.
Harry Harrison did the "Stainless Steel Rat" series which is awesome, not as preachy as his "Deathworld" series. He also did a series about an alternate Earth where dinosaurs got sentient and learnt to breed biological alternatives to our motorized society, like algae that poop steel into seamless bands, and giant itchysaurs that can haul cargo across oceans--and the human beings that fight them.
The core joke of Hitchhiker's Guide to the Galaxy is that of course no civilization would develop personal computers with instant remote database recovery, and then waste this technology to find good drinks.
Steve Jobs has ruined this joke.