for me it is Tsukihime (if you enjoyed fate and especially it's darker side this will be right up your alley) it is made by Type moon as their first VN and definitely shows it's age now; but if you can look past it's technical issues it is truly a masterpiece. I read the original VN which was released in 2000 and my profile pic is of one of the girls from that VN.
there's the "sequels" plus disk and Kagetsu Tohya as well as a spin-of fighting game you might have heard of called Melty Blood. and this is the VN responsible for spawning the creature known as neco arc.
PS: I made a community specifically for Tsukihime and it's directly related media if you want to discuss it [here ([email protected])](https://iusearchlinux.fyi/c/tsukihime) and I made a post going over where to start reading if you are interested in picking it up.