In addition to posting news, a science blog should serve to make science more accessible by being less formal in its presentation. By blogging a personal (and political) viewpoint a scientist can express thoughts he would otherwise be unable to in a scientific journal/publication. A good science blog serve as a discussion hub for scientists and I am pretty sure that blogs in this way will continue to be important information channels both between scientists and out to the public.
Which blog is the best I think is irrelevant, – we need a lot of them. Twitter and Friendfeed are just a bit to shallow for proper scientific arguments to be made, but like blogs serve as an extension to journals, they serve as extensions to blogs.