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.