This is so hard to answer! But lots of people have this question. It really depends on what you’re looking for… and this is just one person’s opinion!
If you want to be in the middle of it all/closest to public transportation = Back Bay
If you want to be near Downtown but in a neighborhood style area = North End (my personal fav!)

If you want more living space, have a car, and want to be more removed from Downtown (not “easy” to get to) = Southie
If you want to be outside the city = Coolidge Corner
If you want a cute neighborhood with charm and are okay with smaller spaces = Beacon Hill

If you want to be on the outskirts of the city but in an area that’s being built up = Fenway