I vote for the eMac. It's the only one of Apple's designs that I find visually offensive. The all-in-one G3 is pretty unattractive as well although from certain angles it looks like a decent updated interpretation of the Color Classic. I can at least see what Apple was trying to do.
The Apple Network Server was a PowerPC machine, although it wasn't a Macintosh. As a server it wasn't supposed to be attractive, but even so it's a perfunctory tower box sitting on an ugly, mismatched power supply section. In contrast the later XServes are stunning.
Of the beige PowerPC models none of them offend me although the late pre-G3 all-in-one models look like a stack of boxes glued together. Even the cost-reduced clone-era Tanzania models - e.g. Power Mac 4400/7220 - look okay from the front. Ditto the iMac G5, which is essentially a giant iPod. I remember thinking at the time that was a logical step from the iMac G4. Apple had been moving from the postmodern translucent plastic look to sober minimalism and the G5 made sense at the time, although nowadays the bezel looks huge. As
For This points out above the G5's chin looks massive.
The original Mac Mini G4 looked great when it was new, but the plastic on the top had a habit of yellowing, which means that most of the units I see on eBay look ugly nowadays, as if the owner was a heavy smoker.
The 17" PowerBook G4 is mostly great - but the keyboard and trackpad look tiny compared to the rest of the machine. I remember seeing the 17" model for the first time and thinking that the proportions were odd. If Apple had moved the keyboard closer to the front of the machine or enlarged the trackpad it would have looked much better.
Stretching the brief, the Vertegri ImediaEngine looks godawful. It was a luggable PowerPC Macintosh clone from the brief period when Apple allowed Macintosh clones. I can't find a single picture of it in the wild so I imagine sales were tiny.
The original, not-a-Macintosh-but-at-least-of-the-PowerPC era Apple Studio Display looks like a generic clone:
https://everymac.com/monitors/apple/studio_cinema/specs/apple_studio_display.html