For 6X6 I use Hasselblad camera bodies which I can also use on my Sinar P1 by using a Sinar/Hasselblad Panel. They work like a dream with the Sinar field lenses from 90mm through to 300mm and when necessary down to F64! The combination also gives me the full movement facilities of a monorail.
For 6X9 I use the Fuji 65mm and 90mm (Fixed Fujinnon) which I find very good for copy work.
But, the full 5X4 has the last word, where it really matters.