Akuma (aka Gouki) is the brother of Gouken, who was the master of Ryu and Ken... Hence the similar techniques. (Ansatsuken art)
Akuma killed both Gouken and their master (as in, Akuma and Gouken's master), after unleashing the Shun Goku Satsu, a secret and sealed deadly technique...
Akuma can kick both Ryu and Ken's ass anyday...
As for Evil Ryu and Violent Ken, those characters don't really exist in the SF storyline...
But video games wise, Evil Ryu has always been weaker than Gouki overall, except in SNK Vs Capcom Match of the Millenium...
As for Violent Ken ("video games wise" again, not storyline wise), I don't know, I yet have to play SvC Chaos...
But since Violent Ken is a hidden character in this game, while Akuma is available right from the start, it wouldn't surprise me if Violent Ken was overall stronger than Akuma, in this game...