In my opinion, that would have to be the 20th Anniversary Mac. The ultra-smooth feel, the sleekly laptoplike keyboard, the gorgeous flat-panel screen, the large yet understated subwoofer, and the ingenious vertical CD-ROM drive. There's nothing else out there like it.

The NeXTcube would be number 2. They're really awesome. The magnesium construction and the flat black case color that's so distinctive that's known in paint stores as "NeXT Black." It has a very imposing, almost badass appearance, and its Unix-based OS, OPENSTEP, is very elegant and powerful. The only problem with them is that they're next to impossible to find, and are extremely expensive (for a good reason) when you do.

In response to crayz: I think the SGI O2 looks a bit too much like the Compaq EZ-series to be Most Beautiful, but that's just my opinion.

You wanna revolutionary and beautiful, look no farther than the original NeXT cube. Black. Magnesium. Cubic.
Not only that, you could put the computer somewhere hidden, and you had only one cable that connected it to the keyboard and monitor and mouse.
It had a 68000 (one of the big ones), but that 68000 did more than many Pentium CXVIII do nowadays. If I remember correctly, the OS was Mach-based.
The keyboard also had the brightness, contrast and volume keys.
Steve Jobs thought that floppies sucked, so he only put in a MO drive. Expensive, but very very sexy at the time.

gpoc had one, and it was only the lucky few that were given shell accounts on it. We are talking circa 1994, when the WWW was a fairly novel idea.

Anyway, I cast my vote for the NeXT cube. It had personality, it had charm, and it was beautiful in many ways. Manfully greyscale, even.

But then , I am partial to black computer. I could almost propose the Sinclair QL were it not already king of the Most Original and Beautiful Computer Failure Ever.

The G4 Cube is now here to give the NeXT Cube (apparently the only other serious contender) a run for its money. This device is so subtle and small it doesn't even look like a computer. When I see it pictured with its translucent mouse and keyboard, the outrageous 22 inch flat panel Apple Cinema Display, all hooked together with the bare minimum of sleek, unobstrusive cables, I am consumed with desire and respect. The iMac may have been a bold statement, but this is a true vision of what a desktop computer system can be. Mmmmm.... cube...