I believe it is not correct to talk about UI in common. Any UI consists of independent parts and each of this part should be discussed separately:
Global Structure (buttons arrangement, sidebars and toolbars placement, spaces between elements and so on..)
Visual Look (buttons and elements appearance, backgrounds, colors and so on...)
Animation Effects
Icons (design and global style of application icons)
Symbols (design of global symbols system that act like buttons in any app)
Global Structure:
From Tiger to Catalina global structure was almost unchanged. It was perfect, simple and could last forever. Columns navigation in Finder was unique. Stacks and QuickLook introduced in Leopard where really nice improvement in usability. I still believe that best Spaces/Exposure structure was in Leopard. Later there where some bad experiments around Spaces/Exposure. Later it came to some compromise, but as result we have that useless and distracting sliding fullscreen animation effect. iOS-like Launchpad introduced later remained unusable for me, because it was impossible to quickly arrange icons by name and because minimum allowed icons size was too huge. After all instead of bugfix and improve it, they simply removed Launchpad in Tahoe.
Starting from Big Sur original structure was permanently damaged without any practical reason.
Visual Look:
I liked striped background texture in Tiger. Brushed Metal probably draw too many attention, but it was ok. Titlebar fonts with bevel effect visually added sharpness. Overall everything was well optimized for low contrast and low resolution displays of those times. Leopard/Snow Leopard visual look was like improved and simplified Tiger. Lion/Mountain Lion/Mavericks felt like more desaturated lightweight and simplified Aqua. Everything became grey on grey, less contrast and less visible, so i skipped those systems because stayed on Snow Leo. When i saw new lightweight "content-focused" visual look in Yosemite, first i didn't like it, but later i realized that it looked ugly mostly due iOS7-like system icons. Big Sur was too bright, too rounded and with too many visual noise generated by floating selection rectangles. Many complex apps that use native system windows and sliders now became less usable due that new visual look. Tahoe became even more bright, more rounded and generate even more visual noise between UI elements. The main problem here that Big Sur/Tahoe visual look was taken from iOS visual look which was designed for 5" smartphone touchscreen, but not for 27" display and mouse. It is also interesting to see how default wallpaper abstractions and default desktop colors where changed from nice looking artwork and color schemes to saturated tasteless artwork and color schemes (smells like Windows).
View attachment 2528511
Animation Effects and Sounds:
Overall in all macOS versions animation effects usually looks OK. But i really miss that veru special animation effect in Leopard Stacks. Guess it was too complex and slow to render, so was replaced with simple zoom animation in Snow Leopard. Also i wish it was an option to turn off sliding effect when switch fullscreen mode. Enabling Reduce Motion in Settings is not a solution because it disables other useful animations. Also i have no idea why in last versions they disabled that iconic
Poof animation effect
Icons:
Icons style introduced in 10.0 Cheetah (2000) was preserved up to 10.9 Mavericks. Icons where improved from version to version, so it was great evolution of great idea. Those icons where example of beautiful well recognized artwork made by real skilled artists with real vision. That style could literally last forever. But started from 10.10 Yosemite everything was destroyed when stupid kindergarden iOS7 icons style migrated to macOS. One of the ugliest thing was strange green tint on HDD, Preferences and other metal-like icons. It looked like object was lit by low quality CCFL or LED light. In last versions including Tahoe we can see some unlucky attempts to bring back fine details to the icons.
Symbols:
New
SF Symbols system introduced in iOS and macOS looks nice and works well as universal visual language. One of not so many good new things in macOS.