The Wild Wastes series by Randi Darren / William Arand is tops in my book. Jan Stryvant's Valens series is a lot of fun, too.
I don't know if you're still looking for these but there was one that I saw ranking really well earlier this month. I haven't read it but, judging by the reviews, it's pretty erotic https://www.amazon.com/Digitized-Online-LitRPG-Gamelit-Fantasy-ebook/dp/B07C2QDTMC
Several months later, the Succubus series remains selling high, with it and its two sequels ranking at around #1,000 (a.k.a. they're each pushing around 100 copies a day) while the Digitized Online book fell hard to the #300,000s (which is less than one book a week). And why? I don't know why but I can deduce some symptoms, such as: one's part of a series that has actual follow-ups while the other is the author's only book. Just saying to any padawans lurking through here who want to jump onto the haremlit bandwagon for big bucks: you cannot laze around. You might be able to publish a harem story that'll skyrocket to sub #2k ranks for a couple days and have enough money to pay a few bills, but if you're not following it up with anything or have any other books to your name, it'll fade faster than a dream. You can't cheat your way to success. And that includes trying to game Amazon. The most you can get away with without taking a fist across the ass from Jeff "God-Emperor" Bezos's mech suit is double-spacing. MSE completely lost half a million dollars overnight because he tried to screw the house, but the house always wins.
Eden Redd has done quite a few LitRPG erotica. I've actually got her Lewd Saga Omnibus on my Kindle to read eventually. Honestly, erotica isn't my favorite thing, but it's a market in LitRPG so I was curious to see what was out there.
From my experience, I can say that erotica isn't many people's favorite thing— if you read the /r/eroticauthors subreddit, it seems like every week there's a thread talking about how bored people get from writing sex scenes. The reason why there are so many erotica writers, however, is because it's guaranteed money. If you stopped what you're doing right now and wrote 5,000 words of sex with the most half-assed plot imaginable and then threw a cover on top, you could probably make $20 off of it. Thing is, it's guaranteed money the same way getting a job is guaranteed money— you'll get paid as long as you come on time! When you stop looking at it as "money for writing sex scenes" and start looking at it the same way as any other job, it makes sense why people go for it but few seem to like doing it.
I think it's more difficult to give recommendations for "good" erotica than it is for non-erotica GameLit or LitRPG (or anything, really). Erotica is subjective, just like any other type of writing, but probably even more so because everyone's "this is sexy" buttons are so individual. There's erotica other people rave about that leaves me cold, and vice versa. (Hell, even my fellow erotica writer wife and I disagree on it!) Cranking anything out "by the numbers" will get boring, too, but writing basically the same sex scene week after week, month after month, year after year will become a special kind of hell after a certain point for any writer, definitely.
I write "Harem" books, but they are very much in the Erotic category. I used to be an erotica writer. Sue me. LOL My "Battle for Glory" books actually well pretty well. They are definite in the LitRPG Light category. Personally, I find most Harem too tame. As long as they have a good plot, with lots of action, I don't NEED them to have sex. The writer can cut to black, but they'll get an extra star if they have good, graphic sex. It's a fantasy. It's not for everyone, but I happen to like some hapless guy in our world to get lucky in a VR game and score a lot of girls. Honestly, as a writer, I find it gets complicated if he gets more than two or three girls.
It'll sound weird since erotica isn't my normal go-to, but I'd also rather have a well-written sex scene than a very awkward one, or a fade to black. Mostly because if it's erotica, I'm expecting it to be sexy. It's like going to a horror movie and instead of a monster/villain, you get...fluffy bunnies or an awkward comedy. It's kind of disappointing. The problem is a lot of erotica is just written in a very...stilted way? It's like they have a formula for "what's hot" and just adhere to it, usually resulting in some really weird language that doesn't seem very hot to me at all. For example, one I read kept describing the woman's "parts" (sorry, trying to be PG since it's an open forum) as "thin". Thin is a really weird way to describe that particular part of anatomy! I could see tight or narrow even, but thin just doesn't work at all. Same for using "spit" for the male orgasm. Ewww, spitting is not a sexy image (to me anyway); far more comical. I believe it was @CheshirePhoenix that mentioned some older books that had a really good flow into the sex and nice descriptions that went well with the rest of the writing. That's the kind of erotica I personally do enjoy when I can rarely find it. I enjoy it more as a good story than a turn-on though so maybe that's why my wants in that area are a bit different.
Rules-Free VRMMO Life is a slave harem story, involving a guy who decides that, after years of playing the Paladin, in this new game he's going for an Evil Campaign. The guy does over the top things, captures pretty women, has his way with them, and has literally dozens of spawn running around by this point in the story. Into the Black is a sci-fi story with a slave harem, but with a more 'Han Solo' type than a demon overlord type MC. He has a lot of fun, and has currently actively participated in making two formerly technological worlds into post-apocalyptic lands by unleashing nanite plagues that eat technology, so both worlds were set back from spacefaring civilizations to the Reinaissance as far as tech goes. Lewd Dungeon, on the other hand, is my Dungeon Core story, and is about an evil bastard who becomes a dungeon, giving him the power to be even more of an evil bastard. One of his major traps in the dungeon involves aerosolized roofies that don't wear off until you have sex. Of course, this is in the middle of a swamp, full of slimes and murderous plants, so...
So is a lot of the harem Dub-Con (Dubious consent)? That's frowned on in romance, but is it prevalent in fantasy/litrpg/harem? Gor was a lot like that back in the day, IIRC.
Yes, a lot of Harem is dubious consent, or straight nonconsent. There's a lot that is completely consensual, of course, especially in Japanese Light Novels, but yes, harem as a genre tends to be either high school (all the girls fighting over a guy who may or may not be oblivious) or slave master (guy rules the girls).
Having just recently plunged into litrpg, I've discovered quite a few decent audiobook productions available, if I could find a decent Text to voice narrator, I could definitely exceed the limits Audible has to offer. Don't get me wrong, I drive for 8 to 11 hours a day and Audible is extremely useful in helping me fill the most boring and mundane parts of my day to day. But, there are a few litrpg series available for audio with only the first two or three books finished. Cherry Blossom Girls is available upto to book 3. Gun Meister Online, in my opinion was really good and is in dire need of a follow up. Super Sales on Super Heros is going very well so far, I'm halfway into book one with book two already purchased, but I don't believe the third is up for audio yet. The Apocalypse Gates series has me left on a cliff hanger due to only the first two of the collection being available on Audible. I know, I know, it takes time what with the production and all, and there's at least a couple of studio's that really exceeded my expectations, multiple narrators for masculine and feminine characters, even running water for a shower scene, not bad at all. In my opinion strait up erotica is really boring, it's xoxo, squish squish, splash. On the other hand, Vortex: Harem at the End of the Universe is performed by Charlie Boswell, which was a real joy kill, while the plot was interesting, I couldn't stomach round two.
Have to admit I think you hit it spot on with most erotica. There are some authors that can do it well, and have a great story too. The biggest issue I have isn't about harem books themselves, but mostly that a great many writers lean on overplayed sterotypes, tropes, etc. It gets boring if the characters are all shallow archtypes, and if the books focus more on constant erotica instead of any sort of plot advancement. Seriously, let me use Alpha World for example. Don't get me wrong, Daniel Schinhofen is better than most harem writers, book's one and two of Alpha World had a decently flowing plot, but book three was mostly the same plot recycle over and over again. Kill stuff, and pretend that the harem characters actually have a chance of dying, then have an erotic scene in the tent every night, then a brief action period, followed by the same thing over and over again. Seriously, I had to grit my teeth by the end of the book when I compared to how well that sort of thing was blended together in the first two books. When the writer pretty much started doing the same thing in the forth book I just deleted the books from my reader and moved on. Its a pity, because I thought his worldbuiling wasn't bad, just that stupid feeling like groundhog day in each and every chapter. If you want a really good example of well done audiobooks with erotica scenes I advise checking out the graphic audiobooks, specifically the Deathlands,Survivalist, and Outlanders series. They are done very well, with good voice acting and everything. James Axler, in particular also has a good balance between the erotica and action, and each scene isn't just the same thing over and over again.