I used to prefer Google's e-book store, because of the partnership with local bookstores (which is now ending, as Synamon said). I expect that I'll continue to buy from Google, but will probably increase the percentage of books that I buy from Feedbooks, Kobo, B&N, and Books on Board. I've purchased books from all of them, and have been very satisfied. I strip the DRM from everything I buy, so that's not an issue for me.
I should also mention that I have a favorite European source for things that are otherwise unavailable due to geographic restrictions, but I'm going to refrain from naming them lest someone kill the goose that lays the golden eggs. :-)