why is Mr. Frank a logical choice as leader for the group in the secret annex

Mr. Frank is the logical because the annex is located upstairs from his business. Otto Frank opened his annex to help people.... something he didn't have to do, but something his moral goodness could not refuse. It was Frank's employees that ran errands, provided news, and supplied the people in the annex with necessary items.

Mr. Frank is the best leader because of his qualities he is honest, he is good at making decisions, and because of his capabilities. Mr. Frank is a good leader also because he helped the Van Daans and Mr.Dussel.