Korosensei's interaction with Karma at the end of chapter six, helping Karma take a turn for the better.
YMMV, but many of Koro-Sensei's interactions with the students can lead to this. Some big ones include Koro-sensei protecting Nagisa in Chapter 1, then in Chapter 2 when he helps Sugino realize why he is struggling with baseball, then again in chapter 7, with Okuda who had trouble with anything but math and science.
In Chapter 11, Assembly Time, the Student Council purposefully didn't bring enough schedules in order to humiliate Class E. Koro-Sensei quickly HAND WROTE and distributed copies for each of his students.
And later on, when Nagisa is being bullied, Koro doesn't intervene specifically because he knows that Nagisa won't be intimidated by the likes of the higher-ranked students. He's so confident in this fact that he shows his smug striped face. And he's right.
Chapter 10, when Bitch-Sensei insults the students and tells them to not bother studying for High School exams (because she claims they will more than likely fail) the students go berserk and demand Koro-Sensei back. This is actually rather heartwarming and awesome when you realize they actually do like Koro-Sensei and actually like his lessons (or at least prefer them over Bitch-Sensei).
A little YMMV but when the rest of the class expected Ritsu to take out her guns she, instead, took out four bouquets of flowers that she promised to make, proving that Koro-sensei's upgrades allowed her to obtain free will and disobey her creators.
Chapter 14: Test Time. Initially, the test questions are shown to the reader as gigantic, terrifying monsters. Then, as the students of Class E remember the lessons that Kuro-Sensei taught them, they transform into something weak and harmless. Up until they hit the questions that the Chairman had put in intentionally to trip up class E, they were doing extremely well. Had it not been for the Chairman's interference, they might very well have all reached Class A! Whatever you might say about Koro-Sensei's unique methods or his unusual appearance, he is an EXCEPTIONAL teacher.
Chapter 90. Gakushuu challenges Class E to a strength contest because he discovered Isogai's part-time job. When Isogai tells the others not to worry because he's gonna take full responsability for it, the rest of the class, who has done nothing but praise and admire him in the first part of the chapter, calls him an idiotfor thinking he would be alone.
Chapter 91. Koro-Sensei being a doting parent for his students, completed with him taking pictures of everyone, then helped Isogai to put on his bandanna while encouraging him.