Next month will see the release of a new
CSM Codex, meaning their rules might change significantly over the index we have right now.
That being said with 8th edition the helbrute has become useful for the first time, as well as Chosen.
CSM Lords have always been one of the strongest
HQs and they complement Chosen very well now. That being said, the Chosen in the dark Vengence box have a very unusual weapon choice.
Cultists... there are a lot people here who find them useful, for me they've always been a waste of points. They can hold an objective, but they won't kill anything and they will die whenever something shoots at them. And they aren't as cheap as one would think.
For a guideline, you should ask yourself which Legion/ renegade warband you like. All of them like to concentrate on some aspects of the
CSM codex. So do some reading in the lexicanum and find out what you like about
CSM, be it a paint scheme or certain units or a Legion. From there you can start to enlarge your collection. Welcome to the Eye of Terror