I'm a huge JBJ fan.
I'm a huge defense fan, especially up the middle.
I love JBJ's defense, but I don't think he is the best CF'er in MLB. I'm not even sure if he's top 10 anymore. I would think, based on my own observations, which is severely limited on non Sox CF'er observations, he is top 20, but I'm not sure. I see a lot of very good to great CF'ers these days. Many are making great plays against us. Perhaps JBJ has slipped to the bottom 10, which may mean he is close to "replacement" level on defense. Many AAAA OF'ers' only skill is defense.
IMO, JBJ's defense is still better than "replacement," but I think it has slipped. The dWAR and UZR/150 numbers are flawed, but they are still better than my own opinion. That doesn't mean I think they are right on everything. I have always claimed they are not perfect.
I stick to my point, and I don't think little leaguers are as good as JBJ, but my guess is many AAA CF'ers can field very very well.