I wouldn't use the word beautiful for any of them, but there are some that are very visually appealing.
I think that a mint Tron is one of the best looking machines. Unfortunately, I see that game as a fine example of style over substance.
I love the look of Joust, mainly because of the sideart.
Defender represents the epitome of a classic arcade machine to me. Plus the design is sharp, angular, aggressive; you might even say "badass" (the game's reputation for brutal difficulty reinforces that image).
Asteroids has a cool, very classic looking cabinet design; it is a good choice for the KLOV logo.
I like the short and stocky look of Missile Command, along with its tinted 'glass, cool sideart, volcano buttons, and large black trackball.
Centipede has great sideart, and is another classic cabinet design (I don't care for the game itself though).
Star Wars is up there with Tron with the cool "tech" look; both the upright and cockpit versions look awesome.
And who doesn't love a classic Nintendo cabinet? They all look great, but I'd say a red Donkey Kong is my favorite in terms of cabinet appearance.
I can't forget 720 Degrees... I mean, it has a boombox for goodness' sake. That's tough to beat.
I could keep going and going. It would probably be a shorter list if I named the ugly ones (e.g., Frogger).