I've always had a soft spot for Metro design, aka Windows 8 UI. Win8 as a whole was a mess of course -- but looking beyond the glaring flaws, I think the new direction captured some of that Win95-esque spartan functionality while also having its own sense of personality. The modern minimalist UI hellscape is miserable, but overly skeumorphic designs like Win7 Aero (or Apple Liquid Glass) are obnoxious and cluttered to my eye, and I think Win8 Metro is an interesting attempt at a best-of-both-worlds.

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.

I thought Windows 2000 Server, SP4 was the best OS for general use. Everything worked, and was in the right place.

The previous version of iOS (iOS 18). I'm not sure when it started (maybe iOS 7), but that and then Android following suit was a great UI.

That said, I'm also one of the minority that actually like the new liquid glass aesthetic.

iOS 4-6. But I understand why they had to abandon it. The pixel perfect bespoke design language wasn’t responsive - ie it wouldn’t scale to screens of different sizes.

I miss the UI for the Playstation 3 and the Playstation Vita.