Rudy Tomjanovich. Former head coach of the Houston Rockets.
A great coach leaves a legacy that's more than just numbers. On top of victories and championships, a coach is defined legendary in the sense that his winning gives an irreplaceable and long-lasting meaning to the city/school the team represents.
Before Tomjanovich coached the Rockets to back-to-back NBA championships in 1994-95, the city of Houston was labeled "choke city" with a long history of failures by its sports teams. The Rockets then, under Tomjanovich's leadership, had to overcome tremendous obstacles in those two years to win back-to-back championships, thus erasing all the dark past and earning the city the new nickname: "clutch city." The quote Tomjanovich left after the Rockets won the second title "Don't ever under-estimate the heart of a champion" is still widely used today, another sign of a legendary coach.