Any where from the box seats to the grandstands are a good place to sit.
Fenway is small, as stadiums go, so there are very few "bad seats.
The grandstands have support columns that block part of the field, but unless you're struck right behind one, it's not much of a problem. Loge and field box seats don't have that issue.
About the only place you don't want to sit are the right field seats. They face right field, not home plate, so the sight lines aren't great.
I usually get there at about 3:30-4:00. that's plenty of time to wander around, get a bite to eat and a few beers.