To me, the worst-looking of the "really old" consoles would have to be the Coleco Telstar Arcade—that's the triangle-shaped one with a gun, a steering wheel, and paddles built into it. It must have taken up a ton of shelf space in stores. Of the newer consoles, I'm afraid I'm also going to pick on the Jaguar/JagCD combo (hey, I own and love them both, but let's be honest ... making the JagCD a different color was the only way they could have made it any worse as a cosmetic match for the Jaguar).
Just to be a contrarian, I'd also mention
the original "fat" PlayStation 3: I don't own any modern consoles, so I only got the chance to see the PS3 in person fairly recently, and I was aghast at how ugly and clunky-looking it was. Perhaps the newer models are nicer.
As for the "best-looking", I have a special fondness for the Atari-style "pedestal" designs, so I'd have to pick the Atari 5200 and 7800. The 5200 is a little too big, but that machine in particular has
style, especially if you pair it up with the incredible 5200 Trak-Ball controllers. The 7800 is a nice-looking machine, too, but you can tell that Atari was very aggressive about cost-reducing that system, at least compared to the "luxury model" workmanship of the 5200.