The last time I went to a game, in June, I sat up in the Coca-Cola corner. It is a great vantage point, you can't see balls that go down the left field line and bounce off the wall, but you have a bird's eye view of wall ball doubles and home runs that go out of Fenway.
It's a unique part of the park. I would also suggest the field box or loge box on the first base side, especially now that they have the netting up, so you can let your guard down a bit. If you sit there, you can see everything, except for the extreme right field corner, but you're in close proximity to the Red Sox dugout.