Babysitter - Incest story between an Uncle and the Niece who comes to live with him. This game has lots of story and plenty of variations based on how you approach things. Be warned, it does use rival love interests as the antagonists but it is also very careful about signalling when you're gonna do something that give the rivals any ground. I go to the writer for this game when I'm looking for advice on my own writing.
Parental Love - This is a very similar, mechanically, game to Babysitter, in it you play a father who has been separated from his wife and 2 daughters for 16 years, due to a drug addiction. You got yourself cleaned up in that time and eventually took over your family's restaurant chain. Returning to your family is a bit of an experiment, but one that happens fast, and woo boy does this game get to the point. It's one I always look forward to.
My Sweet Neighbor - Another one that follows almost the same mechanics, this one puts you in the shoes of a recent divorcee in a new city. Your hot neighbor and her daughter make up the initial love interests for this story, though I hear tell that the future has more in store. This game is always interesting and does a good job with the pacing, while avoiding forcing you to play into the cliche perversions of most other games.
Depraved Awakening - This game is the first one where there's a goal beyond having sex with people. In it you play a Noir Style detective trying to unravel the mystery behind the death of your client. Along the way there's plenty of sexual fun to be had, but be careful, decisions you make have consequences.
Sisterly Lust - This one is somewhat unique, it is almost a hybrid between a game like the first three and a Life Sim, like Summertime Saga, if you have played that. You have lots of options to proceed, a lot of sexual conquests, and plenty to do, but it's also linear, and anything you miss, you miss. Each update is enjoyable, with plenty of sexy content.
I Love Daddy - This was my favorite game for a long time just because of all the interesting turns to the story depending on your choices. It has 10 different endings and I seriously recommend them all, as each one reveals more and more about the plot.
Coceter Chronicles - This is my absolute favorite now. You play as a half demon half fae girl who, in her world, is a second class citizen, but it is revealed along the way that you're much more important than that. This game is an RPGM, but don't let that scare you, it's quite entertaining, and each of the maps are full of places you can go and sexual experiences you can discover. This game also has what I call a "player avatar" thing going on. Tabitha, the main character, does what you would want her to do. If you don't want her to like the taste of cum, for instance, then she doesn't but if you do, then she does.
Long Live the Princess - This one is more of a Life Sim, but with a deep plot and lots to explore. It's use of clues and it's mystery mechanics are actually something I've adapted into one of my own games because I like how it works. There's plenty of options with this game and a lot of fun to be had.
I could probably give more, but these are the ones I'd consider "must plays"