I prefer
Clara. Its more palatable to the general public who still seem to put
Greta in the same category as
Helga,
Hilda, and
Gretchen.
Second I would say
Greta because I like it.
Third
Nina because it always looks like a nickname to me and not a full name. It also has a sort of prissy vibe to it like
Coco and
Fifi.