Top three smut games? That's a bit tough. There are a lot of games I like. There are a few major features I really like, so I'll list those.
- Acknowledging the player's choice. This is kinda mandotory of good RPG and visual novel games with multiple routes and endings. Some games like this are Sisterly Lust, Where it All Began, Hypnosis, and My Legacy. But not just the big things, but little things too - like if the MC is a jerk and sneaks the condom off or something, that should be acknowledged.
- Avoidable undesired fetishes. I'm actually really vanilla. I avoid most games with NTR, Trans, Ryona, Rimming and Furries like the plague. Mlifs and mother/son incest is so common it has become boring. I really like impregnation. Other people will feel differently. But to maximize your target audience, it's best to be able to either turn these features off entirely, or make them obvious in-game so that they can be avoided. WVM does both, by letting you choose if your best friend is trans, and you can avoid the old ugly nurse entirely.
- Animated. Self explanatory.
- Audio. Some games have good music. Some games are completely quiet. It's a nice touch when done and isn't overpowering. Some games include moaning and such during scenes. Try to keep it natural if you implement audio. A good example how to NOT do audio is the RPGM game Manilla Shaw.