What's improved the most?

There were always good speakers, just not so many of them.
There were NO good IC's and speaker cables.
Right now, this very day, a pr of KLH 17's with the Dynaco tube stuff available then, hooked up with oh, Analysis Plus Copper oval wire, would make music on a very high level
and satisfy any rational person, if not a audiopile.

Preamps have made the biggest improvement for me .

Magnat Superflow speaker cable will never be beat, made in the seventies. Same goes for silver Litz ICs.

Speakers get my vote, driver materials and internal bracing and overall cabinet design are the largest gains in audio. The actual music delivery is greater than anything linked in production chain given the addition of digital media from the cd to wireless streaming and servers, the latter not even imagined by most just a couple decades ago.

I think it all has, digital of course has come the longest way because streaming options didn't exist 40 years ago. I didn't even have a PC until 20 years ago. Removing digital, I would say quality and availability have improved the most.

Quality has improved dramatically, across all gear, in all price segments despite the fact that manufacturing methods have become more complex with more options for the engineer/designer. When I was young, it was not a no-brainer that your amp or your turntable would work very long, no matter how much you paid for it. Even good manufacturers couldn't rely on the quality control of the components they used. The iffy nature of it made me more attached to established stores/firms that backed their gear. It was likely that my gear would need service and schlepping or shipping it around was not the easy FedEx affair it is today. Few manufacturers warranted anything. Your town may or may not have had an electronic repair guy who was sober. I once dropped off an amp to be repaired and when I got it back, 6 weeks and numerous phone calls later, not only was it still not working right, it also smelled funny. When I cracked the case I found a cigarette butt in it. Back then, used gear was a real gamble. Part quality and quality control has improved enormously since the 70s.

The other dramatic improvement I've experienced is availability. A greater variety of gear and music and community is accessible to me now than ever before and it is near instant. I can download albums in minutes that I could never even find 20 years ago. Even with vinyl, the change is crazy. I can find an album I could never find when I was young and it's here in days, or overnight if I want! This is just fantastic.

And community! In the towns I grew up in, I was ALONE in loving audio gear. I had NO ONE to talk to who cared about it. As a little kid and even as a young adult, I was limited in range, so I got pretty lonely in this hobby. Now you can commune online. This is the dawn of a great age. I do not take this for granted.