I'm hardly an expert on Dominican hisory, but wasn't he President for like only 6 months? And wasn't he a communist?? (or at least had extreme left wing ideology) which is why he was ousted. That's when the US came in to restore law an order after he was ousted and in the 66 election he was defeated soundly.
Whether Juan Bosch was a communist or not we'll probably never know, that secret went to sleep on November the first 2001. One thing we can do is analyze his brief period of government and by doing so you'll hardly see any policy that was particularly "left leaning".
"His" constitution, although revolutionary by the Dominican standard of that time was not either an indictment on his political ideology. You'll find stuff like; recognition of labor unions, protection of the environment, rights for women and children, reformation of the tax code, school system, the military etc. but really; nothing to go crazy about.
You also would expect legislation like "equal inheritance rights for out of wedlock children" to be no brainers and would expect it to be included in the next constitution (after Bosch was ousted), but not in DR.
Don't get me wrong, he did adopted some policies that could be labeled as "communist"; like securing loans from countries others than the USA (Switzerland, Great Britain) and signing a contract with British General Electric Company to build the hydroelectric dam of Tavera. You see, when you try to gain some economic independence from the 'hood bully you can always be labeled as a "commi".
Was he a communist?, who knows!, but he certainly was a master of the short stories, the Dominican "Chekhov", to whom Gabriel Garcia Marquez used to call: "Maestro". He was also a political genius, a guy who spent almost the entire length of the Trujillo's regime in exile, but after coming back he quickly understood that the country's political fight was heading toward a "clash of social classes", with the "Union Civica Nacional" on one side, representing the upper class, the military and the remnants of Trujillo's regime and on the other side the left leaning "J14" movement. He wisely positioned his party (PRD) in the middle and went after the poor peasants. That's how he won the 1962 election.
Reading his writings on Dominican politics is a thing of beauty, specially when he explains how our folks should be addressed on matters of social and political issues; "you have to talk to Dominicans on simple terms!" he used to say,
"porque cuando tu le dices entra?as, el dominicano entiende intestinos".
The accusations of him being a communist were made popular by a priest named Padre Lautico Garcia, who (Thanks God!) died last year. He used to run his mouth saying that (Bosch) "was a communist and he could prove it!". Bosch challenged him to a debate a beat him to pulp. Right after the encounter Bosch prophetically said that his government would be doomed by accusations of communism. He wasn't wrong.
Now, going back (somehow) to the question of the OP, was Bosch the best Dominican president ever?, I don't think so. First, as somebody already pointed out our "best" president is still to come, Dominicans have never had a good president, only mediocre one. The proper question should be; who has been our least bad president?. I wouldn't put Bosch in that position, his term was too short for that honor, he didn't have the guts to execute the measurements that would have extended his presidency, he wasn't willing to
"darle plomo a la gente", he didn't do what "Lilis", "Mon" Caceres and Trujillo did before him, (and Balaguer after). He refused to rough up the population and very fast gained the reputation within the military brass of being too soft.
His handling of the Haitian crisis didn't help either. For sometime Duvalier (Haitian president, at that time being advised by Johnny Abbes Garcia, former chief of SIM under Trujillo) had been meddling on Dominican's politics. He was planning with the Trujillo's family (and Kennedy's blessing!) to overthrow Bosch. He decided to put an end to that and ordered General Elias Wessin to prepare to invade and occupy Haiti, only to later retract in a fashion that Dominican army men considered humiliating. That sealed his presidency fate and shortly after Wessin disposed of him.
Finally, so you can compare two different style of government, this is the same general (Wessin) who a few years after tried to overthrow another president, but this time he was confronted by a different political animal. He was promptly imprisoned, (president) Balaguer personally read the charges against him on live tv and quickly sent him into exile for many years.