I'll not include mods here.
Gran Turismo 6: basically all cars pre-1990 with a rear-heavy weight balance (RUF BTR or Lotus Europa and the Alpines come to mind). The yaw inertia is very overdone, especially compared to the rest of the cars in that game that all have super forgiving physics.
Forza Motorsport 6: With that FFB... every car is stressful to drive(forces come to late, understeer effect overdone, no natural feel to countersteer), especially in hairpins, off-camber and downhill corners.
Project CARS: Whenever I think "hey, this car is half as bad, drives kinda nice" some minutes later the car shows some unnatural behavior, be it under braking, when driving over a curb, too harsh snap-back from the tires after countersteering... really, most production cars feel "off" immediately, but even the mostly praised GT cars have some terrible flaws and the original setups are horrible as well. I wouldn't know what car to nominate here, they're all somewhat bad. Same if it were "best car".
Assetto Corsa: Outside of the mods, none of the cars are bad in the current build, in my opinion. If had call out a car, I'd say the Ferrari F40's turbo seems a bit too unforgiving (could be just throttle linearity, idk), you'd think that car would be fun, not easy-fun, but fun, for me the power delivery is too hard to control and predict to be fun. I also don't like AC's FFB in the older F1 cars, it's too soft and makes the swimmy'ness much harder to handle than in rF2 or Automobilista for example - still think these cars are well done in Assetto Corsa, just the typical AC FFB, that is fantastic for production cars, is not a good fit for those older F1 cars.
RaceRoom Racing Experience: Very clearly the Formula Junior for me. The clutch on that thing is WAY overpowered, you'd think you maybe even could drive a car like this without clutching somewhat well, but no, needs perfect heel and toe'ing or it's instant death. I don't think it's only my (admittedly not great) skill level, since I love the Formula Trainer in Automobilista and the USF2000 in rFactor 2.
rFactor 2: The ISI Intercepter.period! I don't think I need to say any more. That thing would be a joke if it were a first-time modder attempt, hard to believe it's official content. (Maybe some game update broke that thing? Didn't drive at first release...).
Automobilista: They're all very believable in my opinion, I mean I don't think that the diff and the brakes on the Formula Dirt (that more stabilize the car instead of help you rotate that thing around a corner) are a good idea or "fun", but it's believable. Worst car is probably the Ultima 720. The low-rev torque and power delivery seems unrealistically harsh, if it really drives like that and I were Ultima ...I wouldn't sell a car like that to not professional race drivers, it's like asking to get sued by relatives of now deceased owners ;D
iRacing: The prices of the content is too high for my wallet, sorry. I'd probably be ok with the subscription fee if I had the time to keep the schedule and practice in between, but even R3E content prices are hard to justify for me as a student (which I currently am again).