All the scenes involving Mike and his daughter, in particular the visions of her in 1408, begging him to let her stay with him forever and not to let the room take her back - and then she falls limp as he's hugging her and crumbles to ash a moment later.
Best summed up in just one line: "Daddy... don't you love me anymore?"
A small scene is when Mike goes on a date with Lily after The Room tricks him into thinking he's left. It seems like things are going pretty well, but then Enslin asks if he and Lily could try again to be a couple. Lily sadly turns down his offer...
Mike going to interact with his senile father, after giving him immense crap all his life.