Not quoting or picking any particular post.. just using this as a jumping off point..
I would consider that the MOST IMPORTANT KATA is one of two things.
Both are very ambiguous.
It is either..
1. The very first kata that you learn in your chosen style.
--Reason being. It is the base on which you build all your other kata. The first kata will usually make or break a (potential) karateka. Once the first kata is out of the way and learned, it becomes apparent to the student that they either can or cannot do this. Thankfully, most people decide to continue on, and using that skill gained from learning the first kata, they are able to perform all the other kata that they are taught.
or
2. Your competition kata, if you compete.. this could also be altered to say, "Your favorite kata to perform". You will always devote more time and effort to the kata, any/all of them that you like. However, if you compete and you have one kata that you use for that, it will become the most important kata to YOU. You will give it more of your "attention to detail" and effort.
Kata, IMHO is about perspective. I believe there is room for interpretation in many different kata. Obviously, the more basic kata are less subjective, and have a fairly "set in stone" interpretation. But as kata get more advanced, there are some techniques that become more open to interpretation.. many times based on style, but also based on personal perspective.