If you like SciFi there is the Muv Luv trilogy which is considered one of the best VN series of all time.Wow, this game is amazing, just finished it! The story in this game was engaging and full of a lovable cast of characters. Shame that most games don't have stories as good as this.
I remember playing 999 and Kamaitachi No Yoru like 10 years ago and just remembered those are from the same developer. I have been put off by VNs recently since most are just eroge with simple stories to get different girl routes.
Are there other visual novels as good as this? I am opened to them being +18 if they actually have a good story.
I personally haven't played it but IIRC there are 18+ sex scenes (which should be optional) in the first game, which is also the worst one.