Not sure about the "Best", but
Greydon Square is certainly one of the most underrated rap/hip-hop artists out there right now in my personal opinion. He's a brilliant, very intelligent lyricist with a theme that primarily revolves around atheism, science, religion, and politics. Whoever produces high music has talent too (and high-quality equipment probably doesn't hurt). Sadly most of his songs/albums only have a couple of thousand views on youtube.
One of my favorite of his tracks is called
"Flower Girl" which is one I think the RIU community would enjoy. It strays from his regular theme but it's a good fit here.