My old Z200 was surprisingly comfortable. Comfiest seat I ever sat on was the 1100 version of the Yamaha Virago, was like a sofa but nicer! Mind you that was just in the shop, I imagine it would give me backache if I rode it for more than a few hours.
Edited to add: I imagine the old BMW RT's with the big fairings would be comfy, anything that gets you sitting up straight like in a HGV driving seat is normally pretty good.
Oh and sticking on a small screen a'la GS BMW diverts a lot of windblast away from your chest on the MZ