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.

well... lately it's been Scirocco. I didn't expect to fall in love with him when I watched Zeta but I really, really, really like him a lot. He's so deliciously smug and arrogant in the best way possible. See, when his plans don't go his way he very smoothly goes "ah, oh well" and starts scheming even harder. And he handles other people with such grace... He can gaslight me any day and I'd thank him for it. Both of his uniforms are pretty sexy too.
If you asked me a few years back it'd be Kotomine Kirei. What I love about him is his struggle to be an ethical person and to find meaning in his life... of course, you see how that goes. I love seeing characters struggle with their innate desires.
This is an odder pick: I also, for a period of time, loved Mephistopheles from F/GO. I don't think a lot of people know this about me but I think clowns/jesters are very cool. I love his wacky/bombastic personality and his flamboyant clothing.
Takeda Akane from Hanachirasu, I couldn't figure out my feelings about him but I realized it's less sexual and more "I highly respect this man and everything that he stands for". Well, he's a little bit amoral, but that's fine. I think the way he lived his life and died was very beautiful, to carry out one desire like that so single-mindedly.

Yes, I can highly recommend Muramasa to just about anyone. There's a lot of different elements of it to appreciate: the huge cast of (strange) characters, the battle scenes that are described with point-device precision, the alternate history setting (also described in incredible detail), the atmosphere, the themes... I think Muramasa is definitely one of those VNs you have to play if you have any interest in the medium, even if it doesn't look like something that's 'up your alley'. I think even if it doesn't meet your expectations, it's a very ambitious work that will definitely leave you with a piece of it. And so on. (I'd recommend YU-NO for the same reason)

OK sorry for the 2 month late reply, I saw this question right before I went to visit my friend over at another city and decided to spend the weekend there just thinking about the question... eventually I just forgot to answer it. But I have been thinking... It's just so hard to pick one! So I'll tell you about a few.

I really, really like Dostoevsky's dudes. No doubt people know this if they've talked to me for a while. I think I may have posted about them once or twice on certain places.

From Crime and Punishment is Svidrigailov, who has a huge list of crimes associated with him... He's a very fascinating and enigmatic character despite of how much of a degenerate he is. Well, more like he's fascinating BECAUSE of how much of a degenerate he is. In my eyes, he's supposed to reflect the "extraordinary man" that Raskolnikov describes and wants to be--somebody who is permitted to kill people, even in the dozens, no, hundreds. Svidrigalov is one such man, having almost no conscience and an incredible charisma around him. Whenever Raskolnikov is confronted by Svidrigalov, he's always disgusted by him, in spite of how Svidrigalov is more or less his model of an "extraordinary man". The chapter where he has a long fever dream before he ultimately commits suicide is amazing, as well as the chapter where he confronts Dunya, backs her into a corner, puts his arm around her waist, and asks her if she'd ever be able to love him... When she says "no, never", he's utterly crushed. And to think just moments before, he was completely unshaken by Dunya pointing a revolver at him. I think about those chapters occasionally. Dostoevsky definitely gets it.

Then there's the Underground Man, from Notes from Underground. A pathetic character, but one I could empathize with (for better or worse). When I first started reading that, I thought to myself "yes, this guy gets it! he understands!" in response to him going on a 30-page rant about how humans are irrational creatures that shouldn't be treated as mere numbers. Then, in the second half, you see this guy stumbling around, making a fool of himself in public while simultaneously feeling like he's better than other people because he's educated, etc... I could understand his feelings. In the very end of the book, when he's given one chance at genuinely connecting with another human being, he rejects it (out of fear?) and decides to remain an Underground Man, completely isolating himself from other people... That really got to me.

For something completely different: Lou Zhen Jie from Thunderbolt Fantasy, or just Di Kong when he's in his monk form. There's something about the way that Urobuchi writes about love that picks at my heart. Lou himself is a character archetype that I love, just somebody so utterly removed from normal human values and is suffering because of it (Kirei is another good example). When he meets his "princess", a bloodthirsty sword who charms whoever looks at her, he goes full murderhobo mode and reveals his full power! The man breaks a sword with his KNEE! He goes around happily slaughtering people in the dozens, all in the name of pleasing his beloved princess... and to think, he'd spent most of his life without any meaning. Most yandere characters suck but I can get behind something like this.