The most unreliable car I've ever owned is the 2000 Honda Civic. It's also the only car I've ever owned.
It has suffered from 2 problems over my 15 years of ownership. The rear main seal gasket failed about 3-4 years into ownership. I bought the extended warranty (I was a poor college student back then) and it paid for the whole $700+ in labor and $15 in parts repair (in exchange for $600+ for the warranty 3-4 years earlier).
The second issue I fixed this spring. The exhaust manifold is Y-shaped coming from the engine block. Stress, thermal cycles, vibrations, and rust combined to crack the exhaust manifold in half. The catalytic converter is attached to the exhaust manifold. And there are 2 oxygen sensors in the exhaust manifold. Instead of paying $1300 at the shop, I sourced the sensors from amazon for $60 or so and got a replacement exhaust manifold/cat for $270 (almost $1000 less than the auto shop quoted). Me and a buddy spent a morning in his garage replacing it. Pretty easy to do says my buddy the shadetree mechanic (plenty of room to work in the engine compartment and no conflicting parts to remove to get the job done).
Otherwise the car has been pretty awesome. The only other things I've fixed were routine over the course of 15 years - another blown O2 sensor and a rusted muffler.