The modern relatively toothless version for refugees from the 19th century is the least horrible. The Reconstruction, 1920s, and Civil Rights Era KKKs are all horrible in different fashions.
The 1920s version was the least violent of the three, but had the potential to go nationwide. If it hadn't been for the improprieties its leadership engaged in, it might well have survived as a strong organization into the 30s and become our version of the Nazis and Fascists.
The Reconstruction era KKK was the most violent, but at least had the relative virtue of being single-minded in its purpose. The anti-Semitism and anti-Catholicism of the later versions wasn't there in general, tho the first KKK was so disorganized, there probably were some
The Civil Rights Era KKK was the dying gasp of institutionalized racism. Once stripped of its official support, it morphed into the almost pitiable version of today. Only that it is still capable of occasional acts of violence keeps the modern KKK from being actually pitiable in their pathetic quest to restore a past that never was.
How do you measure the relative violence-levels of either incarnation of the KKK?