I feel like I wrote this post from time to time on Reddit and I think I'll start this tradition here. I'm. a Honor Harrington fan. I've read several other space operas and they always fall short. The three that came close were Lt. Leary, Kris Longknife and Vorkosigan saga. Lt. Leary was nice, but it failed on World building. Kris Longknife also failed on world building and had astronomical levels of cringe with aliens and plot, but I enjoyed it. Vorkosigan saga had better world building and it was nice overall, but the books without Miles Vorkosigan weren't enjoyable. There were other series that I enjoyed: Serrano Legacy, Vatta's War (those are some of my favorites but they were too short), Starship's mage (it declines with every new book), The Lost Fleet (it has a serious plot problem, the plot doesn't move forward), Old Man's War (it was really nice), Dread Empire Fall (also awesome), Teixcalaan (good, but short), Alarm of War (good, but short and pretty generic), Bobbiverse (I read until book 3, it isn't for me), Red Rising 1st trilogy (really nice, but too Hunger Gamish, this whole dividing society into a cast system is getting old), Ark Royal. The Three Body Problem was awesome and, contrary to most series, didn't leave me craving more after it was over. Edit: forgot to mention The Expanse, it was OK.
I think that what won me over on HH was the fact that she is a complete Mary Sue and other character don't fall far from the tree, there is a nice world building, characters die, and there is a ton of action.
On the other hand, there are some long books that I enjoy that aren't space operas. I really enjoy the Dresden Files (because he is cool and it is a long series), I absolutely love Jack Reacher (it is just a nice fun read, it's like a nice Big Mac), I also enjoy The Spellmonger series, and I enjoyed the Riyria. I disliked Takeshi Kovacs (lack of sequence and plot) and I absolutely hate Southern Reach (VanderMeer), and there is another popular sci-fi book that is written as a report, which I also hated. I don't like those very innovative mystery stories where you are trying to figure out wtf is going on or waiting for a plot to start until the middle of the book.
Got any suggestions? =)
(OMG, after writing this post, I see myself as an incredible hard reader to please)
John Scalzi's Old Man's War series was a long and pretty interesting read.
Premise starts out as a "humanity vs the stars" kind of story, but instead of sending young people to their deaths, the futuristic human society instead recruits old people who have already lived full lives. You can enlist towards the end of your natural life to transfer your mind to a (photo)synthetic purpose-built humanoid super soldier body. If you survive a period of time (5 years?), you earn another shot at life and can elect to become a colonist for far away worlds. Most don't get that far.
Your usual "long-term relationship tensions," "humans are always bad guys," "what will technology think of next?" tropes apply.