An excellent question, Geddy. For me, I think it would be Mrs. Fiddymont. I would want to know what she meant exactly when she said she stood in her doorway "off and on." Did she make a pot of tea at some point or go pee? Did she attend to her sick husband and numerous children? Did she have a clock in her home that she could see from her doorway etc.?
I think her responses could undermine Schwartz's testimony or show that she simply missed seeing Stride's murder.
Seeing as G. H. has already been chosen, the next most important witness is Sarah Lewis. In view of Hutchinson's story, Lewis could be questioned further in a more direct manner to describe in more detail who/what she saw in Dorset St.