>>96351023
Abominants are good for rolling up the field and vibe-checking enemy units.
Rampagers are the embodiment of what every CK player wants Chaos Knights to be. Nothing survives its charge.
Desecrators are your AT choice, because most of what you're going to be fighting will out-range your other knights.
Ruinators are bad. Not really worth it when you could use a despoiler.
Despoilers are platforms that allow you to fit two of the same weapon type on a Knight chasis. Two gatling cannons give you 36 shots. Very scary.
Tyrants are used for fighting against deepstrikes and generally just shitting out damage at range. Niche pick for most things.
Acastus knights are cool but stupid expensive and not very effective.
>Which / what should I build?
Ruinator gives you four different loadouts.
The Magnet Baron also sells a kit that allows you to magnetize the kits without needing to cut anything, look into it.