To find a strong contender, you'd need to look at recent years' records, league championships won, and individual player accolades (All-League, All-State selections, etc.). You might also consider the overall athletic program's reputation and resources dedicated to softball. Websites for schools with strong softball programs will often highlight their achievements.
Instead of a definitive "best," you'd be better off researching schools known for strong athletic programs in general and then specifically examining their softball team's recent performance.