Accurate shapewise or realistic behaviour wise? Shapewise, the burnside special is prety good, as for damage I'd say that all of them are pretty good.
They are all accurate, though the more recently made ones (not to be confused with model year) have slightly more detailed/in-depth jbeams
Finding the worst jbeam is easier imo: the pigeon. It had to be a scratch made jbeam because of the shape/size/three wheels, and it clearly shows (maybe because I have worked with it the most). It is very tough and bouncy (not the suspension, the body is bouncy) but this could be explained with it not being made out of metal. However, the jbeam is very simple, most of the beams have the exact same properties so there are no special beams. It's all the same basically, when it definitely shouldn't be for realism. It is also missing common wheels (not that it would be practical), and the suspension starts shaking when parking brake is on and the body takes a hit. The pigeon jbeam is very accurate shape wise for sure. The only problem with it is the right side mirror, the only mirror, where the nodes don't align with the mesh at all.
It's inaccurate because the Pigeon is too light IRL. Lighter items are too hard being Jbeamed, as they will tend to have bugs easily.