Airnimal Joey's have to be the stiffest folding bikes I have ever tried personally, the aluminium makes it almos a little too stiff in terms of road vibration over long distances.The Bike Friday NWT is probably the second stiffest folding bike I have ridden and is steel.
The Tikit is slightly flexy, but still performs really well over long or short distances, and it is 100% CoMo steel.