The 250mm f5.6 Sonnar. This lens needs to be stopped down only to increase DOF - that is providential, because otherwise the maximum opening at f5.6 would be a drawback.
About other lenses which I use, all of C type, 80, 120 and 150mm are anyway good at full opening, but IQ increases almost a little by stopping down - particularly in 80mm. 50mm Distagon needs a generous stopping down to obtain good IQ at he edges of pictures at middle and short distances: the consequence of retrofocus scheme in MF, I suppose. Maybe things go differently with CF floating elements type, but I have not experience of it.
Anyway, in "average" photography of every day, never mind too much about IQ related to opening: remember that you are using Zeiss lenses, "feel" your subject, enjoy your Hasselblad gear and shoot, shoot, shoot!