so many here have completely ignored the question. It was NOT what is most important, it was what should come first.
Nothing is "most important" because it is a SYSTEM. It is the weakest link idea. The SYSTEM can be no better than the weakest link so again, nothing is the most important.
However, we can pick a logical place to start, and that is most definitely with the speakers. They must fit the room and they must be capable of doing what you want them to do i.e. what do you value most because unless you have mega bucks you can’t have everything. You want to play AC/DC at live levels or more interested in string quartets? etc..
Once you have speakers that will be able to do what you want, and that will work in your room, then you can start figuring out what kind of amplification you need to get them to perform, and then you can focus on getting a source that is at least as good as the rest.
So once and for all let’s just drop the ridiculous idea that one part of the system is more important than any other