I'll probably forget loads, but hey.
Ruth Winters' suicide attempt, the music made it even sadder too. I didn't find the sectioning quite so effective, possibly for other reasons anyway, but that was very powerful too.
Megan's death - I didn't even know her as a character until the Sport Relief(?) special episode, but it was so sad at the end.
Patrick's death was not particularly 'sad' or 'disturbing' for me in that it actually made me sad. But it was sad because it was Patrick, and he was just brilliant. Holly's final episode was brilliant for the same reason, although again not really 'sad' or 'disturbing' (and obvs she didn't die!)
Nikki's stabbing episode was very well done I thought.
Lots of smaller scenes with Abs I found very sad or effective, nothing in particular, but just a few things - there was an episode where a man tried to cut his hand off because he was hearing voice and he'd been banging his head, and Abs was talking to him and it got to me fsr (it was also the episode where Dixie played the man's wife!)
And Sam's bipolar storyline. A few scenes with that, but for me, particularly the scene where Tess came home and he was lying on the floor by the bed. And he told her that he'd just wanted to lie on the floor because it was safe there, he couldn't get any lower. It sounds a bit odd, but it was very sad. Kinda related to it as well.