Swift: very stiff (the designer really nailed it with this one, doesn't look stiff but it is superb)
Reach: frame very stiff but some minor flex in handlebar area
Moulton: stiff as (but not a folder)
R20: very stiff indeed, mine has some handlebar flex due to my own design of modernising it.
Downtube Mini: typical flex but maybe better than lightweight Dahons (haven't tried a MU though)
Birdy: Typical flex but quite a lot better than Dahons