Disney Doll said:
Copley Marriott or Omni Parker House
I would not rate either of these as "fabulous". The Omni in particular is very dated. There are some good rooms and some horrors. The Marriott is just a basic conference type hotel. It is usually overrun. I work in the marketing dept of a large Boston firm, so we maintain lists of preferred hotels and restaurants.
NB: I am not sure you'll get fabulous for $200 in Boston in September, as it's a peak season (college kids back, leaf peeping up north, etc...). I think you'll need to spend closer to the $300.
For my $300, here would be my choices, in no particular order.
Boston Harbor is marvelous. My firm does events there and the service is amazing. The restaurants are also great. It is not convenient to Fenway, but Boston is such a small city that it doesn't really matter.
XV Beacon qualifies as fabulous, and so does its restaurant, the Federalist. But I think you'd need to spend more like $400.
There is a boutique hotel called Nine Zero on Tremont that's very funky and contemporary. A lot of our firm's visitors like it.
The Langham is an old world choice. It used to be the Meridien but changed hands a couple of years ago. If you go there, get a deluxe room or it will be small. It's very elegant.
The Westin is at Copley and I've stayed there 2-3 times. If you haven't tried the heavenly bed and shower, you should. And I think you can get in here at the lower end of your price range. DD & I did a girls' weekend there and from our room we could see right into Fenway. It was awesome.
Last, but not least, is the Fairmont Copley Place. It's been years since I stayed here, but I heard that all the rooms were recently re-done. They have a resident labrador retriever who hangs out in the lobby with the doormen and who you can "check out" to take walks with. She's a former service dog and her name is Catie.
http://www.fairmont.com/cop/catiecopley/
Good luck! And lucky you to have scored seats in Fenway!