What's a great and successful game to you? Just out of curiosity.
That's a really difficult question.
I play a lot of games, including the very successful ones, but although most of the top earners are ok-ish for fapping, they are not great VN's (due to unoriginal cookie cutter plot) let alone "games". Some try to insert some sandbox elements but it simply adds to the frustration. Why a game like Milfy City did so well? No idea, although I guess that the characters (even though very much tropes) do kind of work.
The GAME that stands out to me is Good Girl Gone Bad. That's one of the very, very few which actually have game play mechanisms due to the many paths. Our Red String is completely next level.
When it comes to VN's, there are a lot which are quite good, but most of those are not what I think are top earners. Some of my favourites: Young Again, Moment of Bliss, Our Fate, Lost at Birth, Oopsy Daisy, but there are many more.
To come back to the topic, Young Again, Moment of Bliss and Oopsy Daisy stand out because the harem interactions are believable and well thought out. The girls are aware of each other and try to steer a somewhat reluctant MC towards the harem (and the incest). When the girls are not aware of each other, it feels like cheating to me.