The Culture Series
"Use of Weapons" and "Surface Detail" are both incredibly dark and leave you with a sense of dread, or foreboding at times. For instance "Use of Weapons" in the Culture series by Iain M. Banks made me feel physically unwell at times, and several others in the series are pretty dark as well.
Yet, even as dark as "Use of Weapons" is, I feel that "Surface Detail" is actually darker on the physical level. However, it did not make me feel as unwell as the first book. The tone of the book goes down a darker path, it is about people being sent to virtual hells, and in that context it is far more explicit, still "Use of Weapons" was darker on a psychological level for me. I think that one thing that you can say about "Surface Detail" is that there are aspects of the book that feel like they have more in common with a horror novel than "Use of Weapons" which is a at its very core a personal drama, and wartime fiction. Still I wouldn't classify it as horror novel as a whole, just that it certainly contains horrifying parts.
The Ender Series
Ender is a strange thing to categorize. I would say that on a whole, it does what it sets out to do. But there is a loss of quality across the board. And it suffers from a number of different flaws that I won't get into.
If it has flaws you ask, why did it make it on the list? Well, the thing is, all books, no matter how tight they are, have some flaws. So it isn't an aspect that excludes it in my opinion.
One flaw with the series is that it meanders around without an ender in sight.
But if you take the books out of their publication order you can avoid some of these issues. This is how I suggest you read the series.
You should read them in this order, staring with: "Ender's Game" (of course), "Speaker for the Dead", "Xenocide" and "Children of the Mind".
These are the best books in the series. Plus they are the closest you will get to serious sci-fi, and they offer the most interesting science fiction in the series.
Once you've read those books you can either stop there, since you have read the best books in the series, or you can pick up "Ender's Shadow" which, if you may find worthwhile. I ended up flicking through it, since it is mainly a Battle School book. I wouldn't call it a great read, and you can definitely tell that this is where the series starts to degrade.
I would call everything including "Shadow of the Hegemon" a sub par Tom Clancy-style which was written by a man with a laughably childlike view of how geopolitics actually work. It is essentially two thousand pages of very shallow, excruciatingly tedious, and something only really worth reading for those who need a sense of closure for the original Battle School team.
"Ender in Exile" should only be read if you decided to read the Shadow series, especially if you have read up to "Shadow Puppets" otherwise you will not have the final plot elements: Achillies/Petra/Bean, to name a few.
If I had the chance to do it over, I wouldn't recommend it, since I feel that it changes the Shadow series' finale more than is acceptable. While as a standalone work does not add anything of note to the plot.
If you somehow manage to reach "Shadows in Flight", you should just stop there. The reason is that, in my opinion, this book is the only installment in entire series with absolutely zero redeeming characteristics. It is also by far the most insultingly dull entry into the series. And that fact that fans of the series had to wait for more than twenty years for the book's release it is all the more unsatisfying. It is honestly so bad that it finally made me lose interest in, what was growing up my favorite sci-fi novel.