@Valka
Read Dune a few years ago. Great book. Did not attempt to read any of the sequels, and I have caught hints that perhaps they are not as good?
It's really subjective.
For starters, ignore EVERYTHING written by Kevin J. Anderson and Brian Herbert (although BH's biography of Frank Herbert,
Dreamer of Dune, is very interesting). Their books are crap, and blatantly contradict the fundamental premises Frank Herbert set up for the series.
Now, for the real sequels:
2. Dune Messiah
3. Children of Dune
4. God Emperor of Dune
5. Heretics of Dune
6. Chapterhouse: Dune
Dune Messiah and
Children of Dune were partly written at the same time as
Dune itself. I have read an alternate ending for
Dune Messiah, in which Princess Irulan is killed by Fremen fanatics for reasons I won't go into because I'd have to give away a major spoiler.
Dune Messiah takes up 12 years after the end of
Dune. The Fremen jihad that enables Paul to consolidate his power and control over the Imperium has accomplished most of its goals, and the novel is concerned with how the various factions are still vying for their portions of power (not to mention there are rebel factions among the Imperial Great Houses plus some of the Fremen tribes). So it's a political power struggle among the Atreides, the Bene Gesserit, the Guild, the Tleilaxu, various Fremen factions - some of whom hate what the Atreides have done to their world and others who are using their past associations with Paul and Jessica to set themselves up as the power behind the religious arm of the Atreides family - plus the exiled Corrino family. And in the midst of all this, there's the domestic arrangements where Paul is married to Irulan, but it's in name only; he considers Chani his true wife, and he also has to deal with his 16-year-old sister, Alia, who if you recall from
Dune, was Pre-Born with a fully adult consciousness.
So
Dune Messiah is more politically-oriented than action-oriented. Some people find that not as interesting, but personally I enjoyed it.
Children of Dune takes up 9 years after the events of
Dune Messiah, with Paul and Chani's twins, Leto and Ghanima. Note that Leto is formally known as Leto II, even though Paul and Chani's first son, also named Leto, was killed by the Sardaukar in
Dune. There's a lot of confusion among some fans who think the character in
Children of Dune should be Leto III; they don't understand how standard naming practices work when talking about dynasties.
Some people love this book; personally, it's not my cup of tea because I honestly do not find Leto to be a likable character. At all. And one thing to be aware of, if you've seen the TV miniseries: the twins in the miniseries are several years older than they are in the book.
There's still more politicking going on as the twins have to be readied to take their places within the Imperial government. The Corrino family is still determined to take back what they consider their rightful place, and are hatching plots. It's a bit hard to describe some of the aspects of this book without giving away spoilers from the second. Basically there's a lot of court intrigue on Arrakis and Salusa Secundus, and one of the major themes of the series is really spelled out: Never put your faith in charismatic leaders. They are only human, and flawed.
God Emperor of Dune starts a whopping 3000 YEARS after
Children of Dune. The God Emperor in question is Leto II, who has been slowly changing into a sandworm during the past 3 millennia. Leto is fully in control of the Imperium, and he's really Not A Nice Guy/Worm. The Bene Gesserit have another name for him: Tyrant.
The ecological transformation of Arrakis from desert to green world is nearly complete, with the exception of enough desert for Leto to survive (since he's a human/sandworm mutant by this time, he can't come into contact with water).
This is a book that the reader either loves or hates. I didn't like it for a really long time. There's a
lot of talk where Leto is yakking away to his servants, various members of his family, the Bene Gesserit, and even himself. It seems heavy and pretentious to me, but there are fans who consider it the best book of the whole series. I will admit that there are some good parts to it, and one character I really like (spoilers!
), but it took many years and a lot of discussions with other Dune fans with more patience than I have before I could understand much of what Frank Herbert was getting at. It's not a straightforward adventure story; there's some politics, but in this one there's a great emphasis on philosophy.
Heretics of Dune begins 1500 years after the end of God Emperor of Dune, and there's been a profound change in the former Imperium. The Bene Gesserit and Tleilaxu are the factions in charge, and they haven't forgotten their hatred of the Tyrant. Arrakis is slowly being transformed back into its former desert ecology, and there are once again Fremen tribes roaming the sands. After Leto's death came the Scattering, where the humans who had been closely stifled and restricted from travel and learning under Leto's rule took the opportunity to leave their worlds and head out into space - find new planets and create new societies.
One of these new societies is the Honored Matres, who for some reason have it in for the Bene Gesserit...
Chapterhouse: Dune takes up pretty much right where Heretics leaves off - no huge span of time on this occasion. There's a power struggle going on among the Bene Gesserit, the Honored Matres, the Tleilaxu, and the mysterious enemies the Honored Matres are fleeing from (they found something or someone Very Nasty in the Scattering...).
Chapterhouse ends on a cliffhanger. Frank Herbert had started the final novel of the series (we don't know what he intended to call it; it's known as "Dune 7" among the fans), but died before he could finish it. In this never-written book we would have found out who the Honored Matres' mysterious enemy was, and there would have been a lot of loose ends tied up concerning the Bene Gesserit and the Tleilaxu. This novel would have partly taken place on a spaceship.
Some fans point to Hunters of Dune and Sandworms of Dune as the sequels to Chapterhouse, but those abominable wastes of trees were written by KJA/BH and are just a revolting mess of contradictions and self-references to their own crappy "prequels" that have nothing to do with what Frank Herbert wrote.
As far as the Dune "purists" are concerned, "there can be only six" (genuine Dune novels).
So... I don't know if any of this sounds appealing. I've tried to describe them as best I can without getting bogged down in too much detail or giving away major spoilers. Be aware that a major criticism of Heretics and Chapterhouse is that there's a lot of emphasis on sex in those two novels - and one of the less-than-willing participants is a 15-year-old boy.
. Space Opera? Space Fantasy? Dunno. 
It had an interesting take on the Dry Towns, from when MZB was writing. There's not a Free Amazon in sight.
