It's hard to say, i've driven terrible cars in every objective sense that are actually quite fun, my old Rover 200 was pretty terrible in a lot of ways but was still fun, same for the Citroen AX when i passed my test.
For me i think it'd be a 2010ish Kia Picanto i had for the day as a courtesy car. underpowered and rough engine and comically soft suspension and seats with zero lateral support meant i almost fell out of it going round a hairpin bend. I was used to driving my DC5 Integra at the time with Reaco bucket seats..
My current works van is pretty close though, 2016 2.0 TDI VW Caddy, i just hate that thing. I guess it's not terrible in many ways but lots of things just drive me nuts. I hate the gearing and the on/off nature of the turbo, i hate the driving position, i hate the way all the controls are the opposite of all the Japanese cars I've owned over the years. i hate the wind noise from the mirrors, i hate it's only got a cargo door on one side, i hate the stupid touch screen radio and how painfully slow the satnav is to load up, i even hate the texture of the steering wheel and the noise of the indicators