While it’s possible to find great books on any platform (free or paid), the platforms themselves matter less than your ability to filter and the information you get from other readers. Learning to use advanced search operators can help you find more of what you need on any platform. For example, AO3 allows you to exclude tags, pair up characters, and set a word count. Google search can also be filtered by using “site:[website URL]” followed by your search terms to only look for information within a single website. Furthermore, word of mouth is valuable. Find a reviewer on Goodreads or a Twitter user who reads in a similar vein to yourself and they can alert you to free books, new authors publishing on free platforms, and even older archives of books that you may have otherwise missed. Essentially, the best sources of information are not necessarily the most obvious, and it is building a network of sources that will provide the most relevant information to you as a reader.
Will it be considered as smut or as romance on Wattpad? There are readers who want closed-door/fade-to-black scenes and there are those who want extremely explicit detailed scenes. Most top-rated stories are in between – they are extremely steamy to satisfy the heat-seekers but not so graphic that others who like to read while feeling the tension instead of reading detailed scenes would be put off. In order to achieve this a writer would have to be extremely good at writing sensually charged scenes and then leave the rest to the reader’s imagination. Anticipation and emotions are more important than details. Meeshawrites is a great example of this kind of writer. She writes extremely sensually charged scenes but they are never graphic. Her scenes are just extremely sensual and therefore very satisfying for readers who are looking for good smut but are also not off-putting for those who prefer to read while feeling the tension instead of reading detailed scenes. The stories that have smut in them and are rated high are likely to have this kind of approach to writing scenes with smut in them. Staying too much in the explicit territory may exclude some readers due to their age or because of their personal preferences while not enough smut may have led readers to expect more smut than what was delivered by the writer under the smut tag.
I'll preface this by saying rankings are so subjective but if you're looking for the rawest form of erotic content with plot to back it up (so not porn), Killing Stalking has sort of a cult following for a reason. The dark twisted psychological horror is wrapped up in a beautiful little boy story and is just masterfully tensioned. Obviously it's not a comfortable read but that's because it explores the topics of obsession and abuse in a twisted form and isn't exactly romance. But if you're looking to be gripped by a very very disturbed dynamic and are in the right headspace to read it, it's definitely a standout read.
For a more recent webnovel, Painter of the Night seems to be getting a lot of love lately. I haven’t read it myself, but the art is apparently gorgeous and adds a ton of value to the story.