Death scenes. All my characters are interconnected. If even the villain dies (which he does) the impact affects everyone; emotionally or otherwise.
Serious scenes.
Lame jokes keep sneaking into my dialogue.
I feel your pain. Smart-assery always pops in at the most inopportune moments for me. I'm just writing along and BAM: smart-ass comment.
Every scene. For me just sitting down and writing is hard. Everything creative I do, just doing it is really hard.
If you mean hard because it involves writing things I don't like/am not good at, then scenes where there's little dialogue but more action I find challenging. I love writing dialogue and it's so difficult to plow through paragraphs of action/thought.
The opening scenes, I find difficult to make something that the audience will want to read on from, to me it's like a puzzle and I can see it in full on the lid but inside I don't know how to put it all together.
If you mean emotionally hard, none so far.