I chose This Isn't Everything You Are by Snow Patrol because it reminds me of how Tris and Four, and even some of the other characters, felt that they didn't belong in their faction, but they still stayed strong about it until they had to make their final decision. While the different "stories" that are told in the song don't really pertain to Divergent or Insurgent, the overall meaning of the song kind of does.
Some Nights by fun. is also another song that comes to mind kind of because you could interpret it as going out to war, and Tris is doing just doing that in Insurgent.
What song reminds you of the Divergent series so far?