The most important component by far is the modem. Without that I am unable to connect to pay the utility bill, without which there is no power, and no system, and the power cannot even be turned back on. So it is definitely the modem. Or any other darn thing I can think of to make the point that there is no one most important anything.
What you do noob, is if you have something that is known to be negatively affecting the sound, or is super easy or inexpensive to upgrade, then that is what you do. Because everything matters, and sorry to offend the others who got it wrong but everything matters pretty much about the same.
Mostly what you want to learn is not so much which things make the biggest difference, because they all do. So forget about that. What you want to learn is which things can you do that will give the biggest benefit over the time you will have them.
So for example a really sweet cart will be a huge upgrade. But depending on how many records you play it may last only a few to maybe a handful of years, at which point your wonderful cart is just about worthless. While a turntable or arm, good ones will make about the same initial improvement but last for years and years. Mine are over 16 years now. I have gone through three carts in that time. If I had bought the Koetsu way back then it would indeed have been a wonderful improvement. No doubt about it. It would also have kept me from being able to buy the table I got instead, which is now with me and running fine all these years later.
So to really get the most from your audiophile dollar calls for planning the likes of which few here can truly advise you on. Once you have the plan, then we can. But by then you won't need much in the way of help. So relax. There is no right. Or wrong. And you will never be done- until you are ready to be done.... for now.