How do I counter Chaos Knight

How to play chaos right?

I can't answer everything, but I can say this much:

1.) How do you play chaos?
As "soup", i.e. cherry picking from as many codices as possible. In fact, the CSM are not doing well at this point.
At the moment I am observing a trend towards "REd Corsairs" because - if you play 3 units of CSM - you get +3 CP. Then run and attack as Chapter Trait. Some players are currently combining that with 2-3 Lord Discordants and Chaos Knights.

Otherwise there is a thread below by DuderusMcRulerich, in which he presents a 1250 list of a friend. You can read a lot about good combos there.

Berserkers in the Rhino are certainly playable, but then you need a lot of good targets that the opponent wants to get away. So for example 2-3 Rhinos with content + Knight; or 2-3 Lord Discordants + 2-3 Rhinos; etc...

"Classic" is Alpha Legion, where practically all models have the Slannesh mark. This then gives improved protection against fire and Mal des Slaanesh helps to distribute more. You can build nice things there both in fire and in close combat.

2.) Which Stratagems?
Those are popular for infiltrating, for double-firing and for bringing back to the cultist. Everything else is situational. I mostly use re-throws because it is often very beneficial, for example if a batch is not made.

3.) Models that you should buy.
You have to decide that on your own. I would never get Abbadon, for example, others see it differently. I don't like cultists either.
But I think bikes are cool (hardly anyone else plays) and I like all my chaos spawns (it's a matter of the models). I also think termies are cool, but they're not good, you don't have to discuss them. It is ultimately your army, so you also have to decide which models to "need" to.