There are two levels of question/answer here, one being logical/algebraic, the other literary. Algebraically, all three sentences mean exactly the same thing: magnitude(terror)=maximum(magnitude(all her previous experiences)) Note that the right hand comparator references the set of all her previous experiences, not just experiences.of terror as suggested by Matt's discussion of 1. In literary terms, 'on a par with' has a clinical or statistical ring to it, making option 1 seem a rather detached and strange way of describing intense emotion; 2 is close to the spirit of the original.

1) Implies a comparison between the *now* terror and the *most overwhelming ever* terror. You are saying that what Sansa feels now is equal in feeling to the most overwhelming thing she had ever experienced, although further elaboration is needed if you want to specify what that other terror, the *most overwhelming ever* terror was. 2) Works well, but if you want the sentence to stand alone change "the" to "this": Nothing Sansa had ever felt was more overwhelming than (this) terror. OR Nothing Sansa had ever felt was more overwhelming than the terror of (this experience).