Not an easy question to answer,because I have and love all of them (Zeiss Z's) in more than one copiy..
They are all magnificent lenses,but...
If I had been pushed to the wall within your specs,It would have been:
ZE/ZF Distagon 25/2.0 - slightly heavier than than the classic ZE/ZF 25/2.8,but also contrastier,sharper and faster at around f2-f4
ZF/ZE 35/2 - the 35/1.4 is a great lens,but rather heavy
ZE/ZF 50/2 Macro Planar for it's sheer versatility - soft corners up untill f8 considered - ZE/ZF 50/1.4 Planar is better as far as IQ at infinity is concerned plus being faster,yet it does nor close down to 1:2 RR
ZE/ZF 85/1.4 Planar - a great lens within it's (very few) limitations.
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