Hmmm, if you just mean adult as in it has R18 hamslam action, I've got those. But I'm gonna assume you mean "VNs that are definitely aimed at an adult audience".

Full Metal Daemon Muramasa is the big one...along with all of Nitro+'s other VNs. You can't go wrong with Saya no Uta or Kikokugai. In my opinion, you should start with something shorter and to-the-point like Saya. Muramasa is great but it's huge and you probably won't want to read something of its length if you're starting out (this is just what I'm assuming.) These stories have action! Philosophy! Conflicts of ideals between characters! And cute girls!

Umineko is fairly unique, complex, and has a 10/10 soundtrack. Some things in it you'll probably see differently as an adult compared to reading it as a teenager/young child (which is what I did, I should probably go reread it).

Suda51's games (The Silver Case, Flower Sun and Rain, etc) I also recommend heavily. They are awfully stylish and have great presentations. You can really dig into the games if you want, like some people do.

I will personally recommend Kono Yo no Hate de Koi o Utau Shoujo YU-NO because I love that game to death. Amazing soundtrack, CGs, plot, and one of the most ambitious games of its time.