Having just completed "collected tales, sketches, speeches, and essays" by Mark Twain (bet. 1891 and 1910), I must again lavish praise on his literary abilities.
Of course, we remember Twain for his Tom and Huck stories, but he should be given better credit for his "shorts". His humor and satire are without parallel.
I could comment on many of things he wrote about, but there's one that stands out in my mind. It's been exactly 100 years since he wrote "Letters from the Earth", which I read for the first time. (They weren't published until much later--1962?--perhaps for their controversial subject matter.)
Twain goes beyond satire to rip "God", religion, the Bible, and religious people. To me, his views on the subject elevates him to more than "just a writer". He becomes a philosopher, a thinker, and yes, a genius.
If it's been awhile since you've read Mark Twain, I encourage you to return to what real writing is.