Which is the most important part of a stereo system?

I think Millercarbon has a strong point. If you change one part of your system it can change everything. If you believe there’s a gap or weak spot that’s probably the best place to start
I recently upgraded my integrated amp - the preamp and amp combo added a DAC snd upgraded my phono stage improving the sound of my streamer, turntable and tape deck. And a friend‘s first comment was ‘I can hear why you like your Sonus Farbers...previous experience was they were analytical and lacking in mid-bass with poor details. Any improvement can improve everything.

I'd call the "synergy" among the components and the cabling the "most important part" of a system.

If I'd put more emphasis anywhere, I'd go with the sources.  "Garbage in, Garbage out".

You hear people say the source? But you also hear people say the speakers. I was a press photographer in College and it's the lens that captures the image on film talking 35mm here. The better the lens the better the image. The body of the camera is essentially a box to hold the film. I would see students in the ’80s and 90s buy the top of the line Nikon for thousands of dollars and shoot with the standard lens it came with.
Eddie Adams the famous Time-Life photographer told me buy a $500 camera and a $2000 lens if you want to capture a good image. I think it's the same with the speakers being the lens and the source being the camera body. Also read that someone was suggested, Marantz I had terrible trouble getting my top of the line $6000 unit serviced after it crashed and burned in the first 12 mo. Has anyone else expended this? I know it's a separate question

The most important part of the stereo is bragging about how much money I spent.

After that, the room.