well, the worst car in GT3...it's probably not fair to nominate cars that aren't really built to be great, but the Audi TT and Suzuki Escudo, i think, are the worst, unintentionally so, cars.
the TT has a "four-wheel drive" system that turns into FF after a certain rpm (very low, i might add). it also has a problem with booost pressure: it drops off a cliff at 5500rpm. the Escudo is, obviously, overkill in every direction (on the road, at least).