ERA leaders with at least 1,000 innings pitched since 1915:— Ace of MLB Stats (@theaceofspaeder) June 5, 2016
Mariano Rivera 2.21
Babe Ruth 2.25
Eddie Cicotte 2.34
Clayton Kershaw 2.39
However, this list - which I'm assuming was posted to show how great Kershaw is - is somewhat deceptive.
Cicotte pitched from 1905 to 1920, so the above list includes only his final six seasons. (The two games Ruth pitched in 1914 are also not included, which raise his career ERA to 2.28.)
I wondered if someone like Walter Johnson, who began pitching in 1907 and had a career ERA of 2.17 in 5914.1 innings, got slighted by this arbitrary cutoff of 1915. Yes. He sure did. From 1915-27, Johnson's ERA was 2.55 (3472.1 IP) Running the same search with Baseball Reference's super-awesome Play Index, I learned that Johnson finishes 10th in ERA.
What is interesting is that if you add five seasons to the search - if you change the criteria to pitchers with 1,000 innings pitched since 1910, thus including more of the Deadball Era - Kershaw drops to 9th. Which tells you he's still a Hall of Fame-worthy pitcher, but it may not be as cool to cite as the original tweet.
On the career ERA list, Ed Walsh is #1 at 1.82. Rivera is #13, Ruth is #17, Kershaw is #24, and Cicotte is #25. After Kershaw, the next two qualifying active pitchers are Madison Bumgarner at #160 (2.96) and Adam Wainwright at #196 (3.09).
So, yeah, Kershaw is pretty damn good. But you don't need to cherry-pick data to show it.