If you are buying tickets on a secondary site (such as Red Sox Replay or Stubhub) rather than directly from the Red Sox, some of what's been said here won't apply -- for example, seats in the first few rows of the grandstand won't be bargain-priced.
In my experience, the best seats for the money on the secondary market are in the loge box down the third-base line (box 160 or thereabouts). They aren't as good as box seats closer to home plate, but they have terrific sight lines, and they're usually available for less than face value (what the Red Sox charge for the seats) on the secondary market.
I'll echo everything others have said about avoiding the right field grandstand -- to be safe, don't buy grandstand tix with a single-digit section number. Also, a lot of grandstand seats have partially obstructed views; personally, I would rather sit in the bleachers than the grandstand unless I was in the first 5-6 rows of a double-digit grandstand section (and again, those tickets tend to be pricey on the secondary market).
There's nothing wrong with the bleachers, by the way, particularly if you're between rows 4 and 20. (The first few rows are affected by pedestrian traffic; past the 20th row, you're too far from the field, assuming you're not primarily there to drink beer and look at coeds and only incidentally there to watch the game.)
State Street Pavilion seats are also nice, and are sometimes surprisingly affordable on the secondary market. The sight lines are good, but they tend to be more like what you'd get in a modern park than what's typical of Fenway, so I would probably not buy those if you're making a "bucket list" trip to Fenway.
