Obviously Wells's "The Time Machine" and "The War of the Worlds" have to be on any list worth its salt too.
Regarding the Dune sequels, if you care to trust my 37 year old self's recollection of my teenage self's opinion, as I remember it the other two in the original trilogy ("Dune Messiah" and "Children of Dune") are pretty good, "God Emperor of Dune" is over-long but agreeably mental, and everything after that's shit on a stick.*
*By "everything" I mean the book and a half I managed to get through before I gave up.
Dune is a great stand alone book, the problem lies with the rest. Dune Messiah and Children of Dune are really the same book. Im not sure why the writer or the publisher cut them up, maybe to get the whole LOTR vibe. But the two books could be combined and read as one. God Emperor is a cracking book and really explains the whole of the original trilogy in respect of Leto, Paul's son. The book is set hundreds maybe thousands of years later (Its ten years since I read them so I may be addled) and still has the reoccurring Duncan Idaho (due to Cloning). The rest of the books do have a diminishing return thought Chapter House I think is pretty good, Herectics is rather poor, so the whole series ends on a relative high. Avoid anything by any other author, as what Ive read isnt great.
Also Id add in
The Mote in God's Eye by Niven and Purnelle, which describes in Hard science fiction the first meeting between humans and aliens, I was surprised that I liked it as much as I did. For Asimov, Id go with the robots series
The Caves of Steel,
The Naked Sun,
The Robots of Dawn&
Robots and Empire which are very good and have a likeable lead character. I also like Heinlen's
Time Enough for Love, which is a discussion of immortality and long life. Sounds dull but it is made up of a number of short tales told by the worlds longest living man, so has the 20th century on discussed. Out of the Banks sci fi novels, my personal favourites are
Against a Dark Background and
Consider Phlebas