Sen mentioned earlier in this thread the manwha "welcome to room 305!" and that one is absolutely one I would recommend as well. Some of the characters might annoy you a lot in the beginning, but it gets better. It's more LGBT than explicitly yuri, but definitely worth a shot ^_^
I do read a lot of manwha, but it all depends on what kind of genre you like. If you want absolutely stunning art and fantasy/drama I'd recommend "DICE" and "annarasumanara."
If you prefer romance centered manwhas then I know "orange marmalade" is quite popular! C:
For a real scary thriller I think "distant sky" is great C: