Having read the book, I always felt Sonny was the best Don given that he was the only one to legitimately terrify the other families.
Had he not be betrayed by Carlo, he would have destroyed them all.
That's the point. For how terrified they were of him, they used his own weakness of being such a hothead and acting on impulses to kill him at the toll station. Literally, his tactics got the family nowhere. Then you look at Michael, who concocted the idea of assassinating Solozzo and McCluskey, and then returned from Italy under the guise of peacemaker as he and his father allowed the other families to take advantage of the Corleone's resources, only to make sure that no one had to fear him in the end because they were all dead. You don't want people to fear you...remember "keep your friends close, but your enemies closer."? Sonny didn't do that.