Oh gosh, I have one from each one of the books
Twilight: when Bella gets the phone call from James... I literally could not sit down when I read that. I kept thinking "oh no he did NOT just do that! what a jerk! >:-O" i totally felt Bella's pain when she thinks her mom is being held hostage; that is exactly how I would react to someone kidnapping my mom (ie NOOOO*wibble*OOOO*sob*OOO!!!!!)
New Moon: Chapter 3. much as I love him, I wanted nothing more than to reach into the book and punch Edward in the face (HA! as if that would hurt him
) "when i told you i didn't want you anymore, it was the very blackest kind of blasphemy" YOU'RE DARN RIGHT IT WAS! Also when Alice and Bella are racing against time in Volterra.
Eclipse: after the battle, when Edward sees Jacob in pain, and collapses. I was so scared that Jane was nearby and was torturing him, or something.
Breaking Dawn: Bella giving birth to Renesmee...I cannot stress enough how glad I am that that is NOT how childbirth happens. I could not stop cringing.