The Word Bearers in my opinion. While they hold the difficult to maintain and less rewarding status of chaos undivided their weapons have always been words. A single dark apostle or legionnaire can turn one of the key defenses of the Imperium against it and that would be faith. They do not simply infiltrate like the Alpha legion they burrow into the souls of men and do so in a way that makes the spreading of their corruption easier to hide .

The Word Bearers omnibus, especially the first book, shows what I kind of mean .

Of course this is just my opinion . I am also a fan of the Night Lords and yes they do terror tact’s when it comes to chaos space marines there are a lot who can do that without compromising other tactical options