Massachusetts drivers are among the worst in the country or somewhere in the middle, depending on the factors that are used to make the determination.On the one hand, Boston ranks dead last among 200 large cities in data gathered by Allstate Insurance Company on the frequency of car crashes.   Worcester ranks as the second worst when it comes to collisions.However, a survey by Bankrate.com places Massachusetts as a whole as the 20th best state for drivers. That survey points to a below-average theft rate and the nation’s lowest fatal crash rate.The takeaway from both surveys:  Massachusetts has the highest number of crashes per capita, but the lowest number of fatalities.