Love In The Forlorn City It's a romance melodrama, very sad, with main couple with great chemistry! Don't be put off by the too quick/forced start of their relationship, it gets more believable and the OTP CHEMISTRY really makes up for it. Oh, and there's NO annoying dubbing over their real voices either, unlike other C dramas! Also its OST/Music is just WOW
Mysterious Summer an anthology series, aka different leads/story per each episode. Dark mystery/horror/thriller; unique and creepy oneshot stories.
Those are the only chinese dramas that I think are good (of the ones I've watched, nomally don't like C dramas).
You may wanna try Love O2O if you are into romance. For mystery, action and a little romance do try Ding Mo's When a Snail Falls in Love and Memory Lost which I really like but unfortunately even Part 1 is not completely subbed yet. Loving Never Forgetting is good melodrama.
For a modern C-movie, I'd recommend the very funny Sophie's Revenge and My Lucky Star as starting points. These films are romance, comedy and action all mixed up and as they are films, a minor investment of time.
The Boss and Me (Shan Shan Come to Eat) is a c-drama in a very similar vein - modern girl striving for love and success in the big city.
If you like historical or romance, I'd try a c-drama series like Introduction of the Princess or Singing All the Way or Sound of the Desert. The first two star Ruby Lin (whom I like but is not to everyone's taste!) but all three have complex stories, fine actors and very high production values for a tv series.
If you've seen the recent K-drama, Scarlet Heart: Ryeo, you might give the original Scarlet Heart a peek. Lots of famous actors and a pretty good story.
For complete ridiculousness, Go Princess Go or Naughty Princess are very very silly.
The Disguiser: historical. Very dramatic. It's dark because it's war. Some people are really dark and cruel! It's sad here and there, a bit in the end I would say.
The Battle of Changsha: Also war, but this drama is less dark, it's actually rather heartwarming because of the strong, warm bonds between the family members. But for the same reason it is partly very sad!
(Nirvana in Fire: The most amazing drama ever! It has elements of darkness and sadness but I wouldn't use those words first to describe it. It's so much more than that. Well, it has almost everything.)
Out of the ones I've seen from the ones already mentioned:
Sound of the Desert: It has sad moments (some I found very sad but not in the "oh no, main character dies!" kind of way) and dramatic moments but it's NOT dark.
Ode to Joy is WONDERFUL but it is definitely not sad or dark!!
When a Snail Falls in Love is a crime drama and as such it has dark themes (people die...) but it's really just a crime drama with some little romance, I wouldn't call it "dark, dramatic and sad".
The Journey of Flower is... dramatic I guess, and it has its "bad guys" but it's too silly to be dark. If you find parts of it a bit sad or not depends on your taste I guess... personally I wouldn't really say so.
Scarlet Heart is a historical romance. Partly a bit sad, not more dramatic than most similar dramas, and not dark!
I've only seen a few episodes of Boss & Me. It's about a girl who works at a company where there is a boss who is very bossy. I believe it's just a romance. I've never heard anyone call it "sad"! And why is it "dark"? Because the guy is so controlling it's kind of creepy?
Dark, Dramatic and Sad. PLEASE watch Nirvana in Fire!!!!! Wah I recommend this so much LMAO. It isn't "dark" I would say. But sad and very deep plot. Love me if you dare is a bit of everything.. funny, love, dark, scary, strange, sad.