Will there be Enlightened spren of other Radiant Orders than Truthwatchers, and why does Sja-anat like Truthwatchers so much?
The reason Sja-anat likes Truthwatcher spren the most is because they are the most willing. And she considers what she's doing offering Enlightenment, not corrupting. And she considers their willingness to be a part of this. Outside observers might consider her methods less... involving less volition on the parts of some of the spren that she touches. They might argue with her on that point. In this case, as it comes with the two Truthwatcher spren that you see in the books, they both went to what they are willingly. Fully willingly to become what they are. They are, you might say, participants in her plans. So that's why she wants them.