I was a valet attendant in college, for about 3 years. Driven many, many, many cars.
By FAR, the worst car I have ever driven is a Jeep Patriot. Drove three of them - an old one, a new one (this was 2008ish, so last gen) and a rental. Every single one was memorable because of some devastating flaw that made me feel like I was going to die driving it to the garage. It's funny to think back to those because I remember them so vividly, for a damn good reason. And in every turn I held my breath.
Notable seconds: The Chrysler 300/Charger platform. Worst with the hemi, since the tranny is so god awful. Never know how important a transmission is until it doesn't even transmit power to the wheels, much less shift predictably. And any talk about an interior might make me sick.
Really, guys - there's a reason they're going out of business. A terrible, truly sorry reason.
EDIT: almost forgot about the SmartCar. I don't really have a problem with it simply because it's a game changer in some ways, but really this car was a mistake many years before it even hit the roads. Bad transmission, bad bod control, bad throttle control. Really no point in commenting about the engine of a small car like that, but it was lethargic. I have fond memories of it though, since I could park it directly facing the curb where all other cars would have had to parallel park. It was a great pick for a valet attendant in the north end of Boston.