Been in Boston almost 2 weeks every month this year - stayed at most of the properties in the downtown area based on availability and prices since I can normally get the government rate ...
I would rank them from top to bottom
(note I haven't stayed at the Renaissance since I like walking to work/places, etc. and it is a bit out of the way)
1) Ritz Carlton - sometime you can actually get a good rate - best city Ritz I have stayed at
2) Longwharf
3) Courtyard Copley - great location and I actually prefer over the full service
4) Marriott full service Copley - very small room - and lounge is being redone and currently in turn is in a ball room on 2nd level
5) Courtyard Tremont - very basic CY - avoid if there are other options