In his novel “Anna Karenina,” Tolstoy writes that “All happy families are alike; each unhappy family is unhappy in its own way.” Something similar can be said about New York City mayors: Successful ones share policies and leadership traits, while the failed ones chart individual paths of doom. Over the last five decades, Gotham has experienced both. The successful mayors — Ed Koch, Rudy Giuliani and Michael Bloomberg — stand tall because they took office in times of crises and turned New York’s fortunes around. They had their warts, but each left the city better off than when he started.