The action scenes? The dialogues? The beginning? Climax?
Nope, none of the above. It’s those horrible dull bits. Because you’ve got to write them so they aren’t dull.
It’s the bits that, if you leave them out, readers are going to ask: “But how did he get to town when the last scene was back on the farm?” or “But how did she know that the boy was named Martin, when we haven’t read a scene where she was told?”
They are the necessary bits that explain the grounding of your plot, yet are intrinsically dull in their explanation. You don’t really want to have to explain, “Well, first he walked to the main road, then he hitched a ride with the farmer down the road who just happened to be going into town to buy some chicken feed even though it wasn’t market day.” Or, “She went to the dressmaker’s, and while she was there, this woman called Annie came in, and she happened to mention in a passing conversation with the dressmaker that John’s son was her gardener and his name was Martin.“
Leave out the explanation, and you’ll get creamed by your readers; put it in and you’ll bore them the tears. They are the hardest bits to write!