Casual Conversation

Each genre has so many... sub-genre it's hard to be specific.

Horror: a great pick would be Stephen King (the dude is both a popular author and an great author, a rare mix), a more classic choice would be Lovecraft (a lot less popular nowadays but most people hating on him have never actually read him so...). King writes both short stories and very thick novels, so you should be able to find something that will suit you. I would say: Bag of bones, The plague, It, Dark Tower (long, but impressively good... Thinking about it, I realize I would not mind rereading it soon), Pet sematary,... he wrote so many great books. Even a few of those he wrote as Richard Bachman were exciting read, at the very least. The last one I read from King was Cell, which is not his best but it was still real good read: people getting zombified and becoming enraged through their cellphones, in the hands of King it can't be bad :p

If you like haunted house stories, I would suggest Shirley Jackson's 'The Haunting of Hill House', a true classic but that still works wonders. Another woman writer I like a lot, a little more disturbing though: Lisa Tuttle.

And then there are a lot more disturbing authors but, us chatting publicly online and risking to awaken a crowd of self-proclaimed censors that like nothing more than to hate (heck, even suggesting Lovecraft is already enough to risk waking them up) and then me not knowing you personally, I would not feel confident suggesting any of those author as they can be really disturbing (like, really) and not suited to younger readers either. Which is sad, because a few of them are also amazing authors that could make anyone fall in love with great writing.

Scifi:

'Ubik' by P.K. Dick is my first suggestion, always is. Dick is the author that inspired so many successful movies and other scifi writers. From Blade Runner to the Matrix, also the (imho, shitty & spineless) series adaptation on Amazon 'Master in the High Castle'. Cyberpunk as a genre is hugely influenced by Dick. Ubik is both funny and smart read, Dick uses it to masterfully play with a few of his favorite themes.

'Dune' by Herbert (the first volume, the following ones are, well, following ones).

Classic: 'The cave of Steel', by Asimov (crime story in a future world with robots, Asimov is the author of the 3 laws of robotics and his robots novels/short stories have been used in so, so many movies). Another one by Asimov: 'Foundation' so much more interesting than the meh adaptation by Apple. 'Martian chronicles', by Bradbury (such a great book, like a lot of what Bradbury wrote).

Contemporary authors: I will happily read anything by Paolo Bacigalupi or Ted Chiang to name just two. Hard science: Greg Egan is my go to.

and anything in between

A bit too vague, I'm afraid ;)