Year one had many memorable ones, like my personal favorite, Atheon's theme. It also had the elusive Psion Flayers theme and End of the Line. Year 2 included TDB (meh OST) and HoW (kickass OST). It fit perfectly with the tone. The Taken were a powerful supernatural force of darkness, so the theme was dark and epic. Year 3 had triumphant music, like Eye's Up. Considering the huge library it had, Year 1 will probably be my favorite until at least Destiny 2's base OST.