re: Is the mini-series best for great storytelling?

It's definitely an effective entertainment medium when executed well. In addition to the two you mentioned (which are among my favorites of any movie/tv productions) I've enjoyed min-series all the way back to Rich Man Poor Man, Roots and The Stand. Some good actor have gotten their starts in them - Nick Nolte, Gary Sinese, and a lot more.

The funny thing is there has been kind of a blending of forms. Movies like The Godfather, Lord of the Rings, etc. have stretched out to multiple movies and longer run times while tv shows like True Detective which you mentioned seem to be leaning towards one season "stories". Heck, even stuff like Battlestar Galactica and Breaking Bad, while they run more than one season, have the a specific narrative to tell.