I think there a few different things being confused here.

There's frame flex, there's slop in the design, there's slop due to poor maintenance, there's negligent assembly, and there's negligent manufacture.

It can be difficult to separate them, especially since some are meant to be features (ie one bike's "suspension" system is another's slop).

That being said, stiffest folders I've ridden are the Mu (w/ SL stem) and the Airnimal Joey. Flexiest are the Tikit and maybe the Brompton (though it might be more accurately described as mushy). However, this isn't to say that all Mu's and Joey's are stiff or all Tikits and Brompton's are flexy. IME, you unfortunately just can't pick a model and assume all copies of it are the same because a folding frame is not a static piece of metal, but a moving part.