Based on just reliability, my worst car is the Crossfire. Since I bought it a year and a half ago it has had the following problems:

Cruise control did not work when bought.
Headlight condensation.
Loose battery cable on battery terminal (caused random electronic glitches).
Could not pump gas into tank.
ESP/BAS sensor failed.
Supercharger clutch failed.
Supercharger clutch pulley broke off shaft.
Headlights crazed.
Could not pump gas into tank Part II (replacement tank had same problem as original tank).
Cup holder refused to stay stowed in console.

There may be an extra problem or two that I'm forgetting but these are the main ones. Fortunately my dealer has been fairly competent and quite easy to work with in getting everything fixed. In spite of the above problems the Crossfire is still the best car I've ever had and I have no regrets in getting it (well, I do regret buying when I did since the price dropped considerably a few months after).

The next nominee for worst car is my '91 Spirit R/T, bought as a leftover model in '92. This car has had more timing belts at 167K some miles (sometimes the odometer quits so I don't know the exact mileage) then sets of tires. Couple that with severe paint peeling and engine problems and you have a serious contender here. BUT - I expected mechanical problems so this is not really an issue with me.

Looks like the winner is my '74 Fiat 127 or 128 (I forget which) that I bought in '78. Did it have transmission problems? No. Was it a wretched little thing to drive? Well yes, but what econobox wasn't in the '70s? Was it falling apart? No. Its sole claim to worst ever was catching on fire while I was in it.