Not originally from Boston, I lived in the MetroWest from 2015-21, right on the commuter rail line, so I was thrilled to be able to get to major league ballgames and back without hassle. I didn't have a very high opinion of the ballpark prior to that, after just a couple of visits over the decades prior. I learned better during my time living there.
It's intimate. Even in the cheap seats out by the bullpens, you feel a part of the game. Some not very expensive seats down the left field line are even better of course. Ticket prices are high compared to other places I've lived, but then Oakland baseball isn't a very fair comp for Boston, LOL.
The atmosphere of a ballpark that is in a neighborhood shouldn't be ignored either. Again, compare to Oakland if you don't get my meaning, LOL. I didn't even bother trying to acquire tickets for the '18 postseason (did I mention prices already?), but I nevertheless took the train in, one of the WS nights, just to soak in the atmosphere, visual highlights being the bridge over the turnpike and walking east on Lansdowne, with the good-luck message on the Prudential Tower.
And in a similar vein, the best thing inside the park, just at a regular game I mean, isn't physical either. It's the party atmosphere. Winning or losing, the fans are there to have a good time. If you've ever gotten used to another ballpark where they have the constant "Let's Get LOUD" messaging on the jumbotron, Fenway is a revelation, with the truly organic cheering that goes on. Some leather-lunged fan getting up and yelling "Let's Go Red Sox", with thousands joining in, is music. Speaking of music, Sweet Caroline is electric - if the Sox are winning at that point in the game, the song is a celebration, if they are losing badly then the song is defiant, and if the game is tight then the song is "buckle up for the finish and let's have some real fun!"
In objective terms, the ballpark might be a dump. I lived in the Twin Cities during the Metrodome era, and we had a saying "the place is a dump, but it's OUR dump." Many happy memories there, because the physical amenities are the least part of the equation. Fenway is nowhere on the same spectrum as the 'Dome, don't get me wrong, but it's not an elite facility in this day and age either. Nevertheless, it's Boston's place and I couldn't love it more. Best single ballpark in America to watch a game. But you all know that already.