Depends on your tastes. There are, in my experience, two major "choices" which require their own dependencies.
If consensual play and prostitution is more your thing, you can't really beat Maria Eden + ZAP + ZAZ. ZAZ and ZAP provide equipment, there are a good bit of homes and addons that fit with it, etc. ME doesn't support Devious Devices which is a big issue with it. Its "capture/forced" slavery aspects are very limited, but, realistic (captured, used, sold or killed). It has a variety of minor perks too that are nice, and sadly missing from other mods, like your master/mistress taking ownership of your home and more engagement in actual events, etc. Personally I love you can get married as a slave to your owner, I would LOVE to see that get expanded upon. ME has the both benefit and drawback of essentially one author working on it, which means it's more homogeneous but also less total content. One person can only do so much.
For non-consensual captured/kept style, Sanguine's Debauchery+ is arguably the best option. It has the huge strength of also working with Devious Devices, which means you can do other content as well. SD+ has mechanics where you basically please your captor with sex, money, and food, either resisting mental corruption or giving into it. It used to be a result of defeat but now I THINK only option is to surrender before death in a fight. SD+ is a pretty developed mod, and ties into many other mods including Skyrimll's various mods; you would definitely get the most content using SD+ and then other mods to add flavor. For example you can be sold into SD+ slavery via simple slavery, or Content Creator's Alternative Starts can start you as a slave, or an Alicia (just search for it if you want an explanation). SD+ also can work alongside most other mods even if it DOESN'T tie into them. Really its only weakness is that slavery under SD+ can be boring and less engaging and consensual slavery is not really a thing with it the same way as ME. Skyrimll's working on that, next update should improve that. I actually tried to pull apart SD in CK once when it would still run for me to attempt to add that, but I couldn't make heads or tails of anything since I don't know anything about Skyrim's mod structure.
Other mods: cursed loot has a dominant follower, although it's not as developed as ME or SD+ (Kimy's also doing a bajillion other things in it, so, understandable--CL covers prostitution, a dozen or so kinky side quest type things, misogyny, combat rape, etc). There's a dominant follower in the "sex slaves" mod, Andrew, but most of his content doesn't fire for me. Submissive Lola works alongside Devious Devices, but, is abandoned and a bit incomplete. Generally most have limited content. You can run pretty much anything apart from ME side by side with almost anything else, I personally use two profiles in MO, one for ME and one for SD+/DD/DCur/etc.
I'd love to contribute or make my own mod but I can't even get CK to start for me. At the moment I can't take on a full big project myself anyway, but I can write and code quite well even though I don't know papyrus. So maybe someday the situation will improve further.