Keep your eye on the new (to 8th) and pretty unique restrictions we have on army builds. Every detachment must be pure in one of the three Obsessions: Kabal, Coven, or Cult
Kabal are the kabalites and archons, Ravagers, etc.
Coven are wracks, Haemonculi, Talos, etc.
Cults are wyches, succubi, reavers, etc.
All three are pretty viable and competitive. Just be sure you're focused enough on one of the three up front to get a real, playable detachment before fleshing out any of the other. Because you can play several detachments in the same army, but they each have to be fully legal.
Then, to your real question: Once you know which of the three paths appeal to you most, it would be easier to help advise you on what models and units make a good list. Though, if you look at what's viable in the Obsession of your choice, the answer is usually: get three of everything and then two more Raiders.