I will always recommend a Sicaran no matter the occasion. It is by far my favorite addition Forge World has for an army and gives high mobility units, that are almost a necessity in any army, some much needed support with Autocannons and some either Heavy Bolters or Lascannons.
Most of the FW stuff for CSM is pretty meh, but looks amazing. The only exceptions out of IA13 in my opinion are the Sonic Dreadnought, Ferrum Infernus to replace the dumb Helbrute, Hell Talon and Blight Drones and a couple models that aren't too different than their loyalist counterparts like the Fire Raptor and of course the Sicaran. And none of them are going to really help against armor.
The Knights do some work but I've never been wholly impressed by them for Chaos. The Loyalists have some cool ones, but the recently released GW and the slightly older FW Chaos Knights are pretty meh. So many points for less damage than the equivalent amount of Obliterators other than the D Assault. Because of how meh I've been with them, I haven't done enough testing on Knights in general to say for sure which Chaos set up is the best.