Your post got me searching: I found a Washington Post article with a nice analysis and a left-right scale of common news sources. Both slate and breitbart are outliers:
https://www.washingtonpost.com/news/the-fix/wp/2014/10/21/le...
Time to update my news reading preferences!
Some of those surprise me. I would have expected to see sources like NYT and NPR as more popular with the right than sources like MSNBC and Huffington Post.
edit: I suppose there might be a selection effect. The large networks are watched by everyone, and the smaller ones are sought out.
Slate.com does not describe themselves as a news source, so apparently not. I've certainly never thought of them as informative.
I can't personally comment on Breitbart's content, but apparently a number of their employees quit earlier this year, believing that the site has lost its journalistic integrity since the death of Andrew Breitbart.