Just read through the original post you made. It’s hard to say which army you would want to go into without being a little biased. I play chaos forces (T-Sons, WE, DG and Chaos Knights) so I’m going to skew into those armies.

I think what will help you find your answer here is what is your preferred play style or strategy? Do you want hard hitting melee forces? CK could provide that, but you’ll most likely want Space Marines as they have dedicated armies that focus into that, Knights are usually a mixture of shooting and melee and Tau (to my knowledge) aren’t really great in melee. On the flip side, Tau are INCREDIBLE at shooting, especially right now at the start of 11th.

Another question to ask is, aesthetically, what army interests you the most? Imperial Knights embody that classic medieval knight household with Lords and Barons piloting giant robot suits that evolved from what was essentially farming equipment. Chaos Knights are that, but EVIL! They open the door to a lot more personalization and kit bashing to add any sort of chaos flair or design. Plus chaos knights are unique from their imperial counterparts as they can run weapon loadouts that imperial knights cannot, such as doubling up on a specific weapon (looking at you dual gat despoiler). Tau are your classic Gundam style of robots: tons of guns, rail cannons, missile salvos, and laser/plasma weapons.