I didn't watch the first 2 seasons because of the ugly 3DCG and some of the story content was cut. I'm currently watching season 3 because the animation finally looks decent enough to bring justice to the manga (+music by Hiroyuki Sawano) And it will cover what is considered the best arc of the manga.

I don't recommend watching the anime. Read the manga first and then decide if you want to give the anime a try.