I really enjoyed the second season.
The character development was brilliant and I especially like how we learnt more of Koro-Sensei's past especially how he cared about Aguri and how he had learnt to protect his students, so that if the students ever get into a situation that Aguri got into then he'd know how to save one of his students.
I enjoyed this season so much more than the first season. The ending was perfect, the way they ended the season, the last few episodes brought tears to my eyes and that has only been possible by a few animes that I've watched. If you're planning on watching a very good action anime with a good character development and untold secrets until the later seasons/episodes, then this is the anime for you!
10/10 overall for me!