There are so many, but the most nerve-wracking was in Eclipse when Bella told Jacob they couldn't be friends. That was horrible. Poor Jacob! The words that he spoke to her during that conversation, were exactly the truth- If there hadn't been any monsters or magic, then they would be together. If Edward had never come back, they would be together and be happy.
I just feel for Jacob. He went through a lot of hurt in New Moon and Eclipse and even BD. Yeah, so the most heart stopping and nerve-wracking moment for me, was in Eclipse. Poor Jacob It made me so sad