While we're talking about cool old UIs, I'll also point out IRIX 4dwm[0] (google image search "IRIX 4dwm" for more examples). It's far before my time, but based on the screenshots out there on the 'net it seems like a pretty good UI for it's time period.
That, and a custom set of applications which might have been written in Motif. There have definitely been better UIs since then, but for it’s time I it seems pretty impressive.
It's very much a shame that licensing on Motif was so restrictive. At a time when Unix desperately needed a standardized toolkit, the wrong moves were made, and fragmentation was the result.
0 - https://en.wikipedia.org/wiki/IRIX