Probably a tossup between a Fiat 131 (I don't remember the year but it must have been early because it was before it was marketed in the U.S. as a Super Brava) and a 1977 Caprice.
The Fiat stuck around for a year and spent as much time in the shop as the 911S that replaced it, while delivering far less fun.
The Caprice was a lovable tuna boat with a massive (!) 5 liter engine and a tendency to stall when turning right. It was triple black with no AC, which was okay in Seattle but untenable in SoCal.

Depends on the definition of is worst. Mechanically worst, as in most likely to explode in a cloud of steam, smoke, metal shavings and viscous fluids, would have been damned near everything my brother owned prior to his 30s: Various clapped out early '60s-mid '70s Beetles, an RX-3 with floor pan holes that would have scared Fred Flintstone, 1980 Cutlass that idled like a Kenworth Diesel and, if you accelerated from a stop too slow OR too fast, would just…stop.
As for aesthetically worst…My mother's school-bus yellow (w/ brown vinyl top) 1976 Ford LTD wins the prize for me. In retrospect, it's not the worst I've ever seen, but the combination of the poor color choice, the sheer mass of the beast ('specially to the elementary-school version of myself) and the fact that it replaced a '67 Camaro that could have possibly been mine at some point make that damned LTD my kryptonite.

While I was growing up, my mother had a red/white Bronco II with manual hubs, a metallic-Tobago-Green Contour with a stick shift, and a Vortex Blue over all-black New Beetle turbo (also a manual). Nothing embarrassing so far.
My grandparents, however, were hit-or-miss. Sure, my grandmother still had an '00 Cross Country when she died, even if it was a champagney colour, and an '89 Volvo 244 that still donates parts to my daily driver. The Eclipse and Sebring convertibles that came between them weren't bad, either, at least not that bad.
However, my grandmother had two consecutive first-gen N-body Grand Ams before the 240, and my grandfather went from a Celebrity wagon to a white/blue Corsica to a white/red Achieva SL before the latter was traded in on the first pre-owned Sable in '02 or '03.
I was only developed enough at the time to be embarrassed by one of those cars – the Achieva. And it wasn't that bad for its time, even with its mildly ponderous styling and ridiculous name. So really, it's not that much of a loser.
My mother's first car, though, over a decade before I was born, was a Pinto hatchback, though… that's gotta count for something.

"in the care of my sister"

Great info, thanks for the post!

I didn't have it too bad when I was old enough to drive but not affluent enough to buy my own car because my dad had a Volvo V70 at the time. But before that he had a boretacular Renault Scenic (it's a family minivan!), and his car history even goes back to the objectively terrible Trabant, which however he loves to praise and defend (socialist Stockholm snydrome?).