Not mentioning the project car that I sold before I got started on, my worst car would probably be....well I really can't complain about any of the cars I've had. The ones that were slow, boring or ugly were reliable, and the ones that were unreliable were fast, fun, and good-looking.
I suppose my first car, '87 Taurus. Bought it for $700, all of my own money. Ran like a dream with 146K miles until I crashed it.

Too bad, was a great car, really.