Really depends on what you want to see/do beyond Fenway. Boston is a very small an compact city even without the T. You can cover a lot of ground on foot.
Quincy Market/Faneuil Hall, Haymarket/TD Garden, waterfront/Aquarium, North End, Copley Square, BU/Fenway, Science Center/MGM/Lechmere, Beacon Hill, Back Bay/Newbury are all tourist friendly.
Pick a hotel that is a direct green line stop between one of those and Fenway. You can also stay in Cambridge or anywhere else and transfer to the Green line from any of the other lines. You can also stay near a T hub like Government Center where multiple lines cross.
You might also want to locate around food. Pick a restaurant or two that you might want to eat and stay near there. Getting to Fenway for a game is too easy to anchor there unless that's where you want to be.
I'll usually stay at the Omni Parker or the W Boston in Copley. Antico Forno and Citrus and Salt are two restaurants. for North End Italian and CS for Copley brunch or Latin fusion.