The worst model kits I have personally built have to be divided into two areas:
General: Any of the MPC flip nose kits, especially the'57 Corvette (the weird, wide rear tires made it even worse) and '53 Ford pickup. I built them in the 80's and hated them. Tried them again a few years ago, still hate them. I do like the regular MPC kits, though.
Specific: Sorry to step on anyone's toes here, but the absolute worst is the Revell 1959 Ford Skyliner. I got it for Christmas as a teenager in 1987 and it still sits unbuilt in 2014 because the parts just didn't work for me. I've tried several times over the years to work on it and it's a disaster.
Specific (runner up): Mr Hobby Volkswagen Karman Ghia Convertible. The body is nicely done, great wheels and tires, but some of the parts, such as the seats and convertible boot are almost rubbery and were impossible to glue down. A poor fitting interior tub that never fit correctly didn't allow for contact with the frame of the car, so it could never be permanently attached. Great tooling, very poor follow through.
Best Model Kits I have personally built: AMT '57 Ford, any JoHan kit, and although it lacks separate exhaust, the Monogram 1958 Ford Thunderbird is by far the easiest and most well done kit I've come across.