Prodigy by Marie Lu!
I absolutely loved Legend so I was really excited to read the sequel. And it totally blew me away! The plot so was so gripping and tense and by the end my heart was breaking. There's plenty of action and I think this trilogy is becoming one of my ultimate favourite YA dystopia/post-apocalyptic series. If you haven't checked it out yet then I urge you to straight away! Here's my full review of Prodigy.