Best, Visually :-
ZX Spectrum original - iconic design, and will never go yellow. Mine still works today, and still looks amazing. And, people are still doing amazing things on the Spectrum like the amazing Bifrost and Nirvana engines.
TI99/4a - IMO, still looks amazing with a sort of 70s silver futuristic look to it - This wouldn't look out of place on a Blakes 7 set. Home of some amazing ports. Has some great modern hardware like Nanopeb, and Flashrom 99.
Oric Atmos - I love the contrasting Black and Red look to it, and it has a few great games. And it never yellows, due to it's colour. Most machines on ebay still look great. I love mine, even got a matching tape deck.
Commodore 16 - Ages better than the C64 breadbin, due to it being black, although the keyboard can yellow. More reliable than the Plus/4, but you still need heatsinks on the TED and 8501. I really love mine, some great games like Big Mac.
Enterprise 128 - Always liked the look of this machine. Never had one, and the prices these go for means I probably won't own one.
Memotech MTX512 - Another, built-like-a-tank computer. I think these also looked great. I remember seeing the adverts in the 80s and wondering what they were like. Only ever tried one at a computer event, never owned one or knew anyone who had one.
Worst, Visually :-
Dragon 32/64 - Never liked this behemoth of a machine, it's just so damn ugly. Looks even worse if it yellows badly. And the colour palette was awful, just green, more green, red blue and yellow. Oh, and green. And the joysticks, oh dear god, the joysticks.
Commodore 128 - Dear god, this was a monstrosity. Complete waste of time and effort bringing this thing to the market, and it flopped. When they yellow, they go full Commodore yellowed.
Atari 1200 - Not a popular choice, but I really think it looks terrible. It's like they took a 800XL - which still looks great, and just slapped a huge Frasier forehead on it. Thankfully, it didn't leave the U.S.