Not only is it subjective, but some of the chains vary considerably by store! Of course the "best" single-store cafe will be considerably better than any chain. I would pick a dozen cafes as having better coffee (starting with the beans, some roast their own). I knock Starbucks down for lack of consistency, some of their outlets have had major quality control problems (not surprising with 8000 outlets).

Tim Horton's coffee is interesting. I think they treat the beans to reduce the acid level. In any case there is a distinct Tim Horton's taste which many people like. Mild, mellow and smooth (and very consistent). Easy to tell apart from most other coffee sources. It isn't my favourite but I have to give them credit for this clever product strategy.