Strictly visually, I'd say the best are:
Original IBM 5150 - not a lot of ABS plastic, probably the most solidly built of any computer ever. I've seen them rust, but they don't generally yellow, and if they're even marginally taken care of they clean up like new.
Atari XL line - the beige part does yellow a bit, but usually not to the point that it's even noticeable. Most of the computer is black, which usually holds up well.
TI-99/4A silver - I don't know what metal it's made out of but it looks like stainless steel, and almost all of these still look like new unless they were heavily abused. The TI-99/4A is the Delorean of computers. (Probably in more ways than one.)
All of these also just have a classic design that still looks good beyond the lack of wear. Some early computers, like certain Radio Shack models, are just ugly to look at now even if they're in good shape.
Pretty much any Apple II - they just yellow so horribly. The IIGS is marginally better than the other models, but not much. The II/II+/IIe also have the problem of losing their keycaps way too easily. I can't believe how many of these I see with missing keycaps. What are people doing that keycaps are just popping off all over the place?
Most Amigas - same problem. I had an Amiga 1000 that was practically orange. And it was unevenly discolored, which made it even worse. I couldn't even look at it. This was before retrobrite, so I actually sold it because of this.
Atari ST - mostly because of the keyboards. The keycaps yellow more than the case, and they always do so unevenly too because I guess they were made in different batches from different plastic.