Armor sets are classified by the type of gear they provide, so first you want to look at what kind of stats your class specializes in. For example, if youre a Titan with a focus on defense and melee attacks, then armor like Sunbreaker or Inverse Shadow will be good choices for you. If its ranged weapons that interest you most as an Hunter-class user though, then consider choosing something from Synthweaver and/or Gjallarhorn sets instead.