For me, one of my favorites was the look of some of the later PalmPilot UIs. They were surprisingly good for a display of only 160×160 pixels. They may not look the best today, but they were very functional. In some ways, I actually prefer older UI designs. Modern UI components feel more complex and, in my opinion, lead to a greater number of bugs.
What are everyone’s favorite UIs, old or modern?
Honorable mention goes to the original Nintendo DS UI. For a console so laser focused on the casual market it's weirdly professional looking. A little on the sterile side but it works great for the low resolution.
That said, I'm also one of the minority that actually like the new liquid glass aesthetic.