Barron's or Princeton Review for ACT prep?

Of the books you mention, I would recommend Barron's ACT 36 as the best resource, especially if you're already a relatively high scorer. The Barron's book has more challenging questions (often too challenging compared to the real ACT) and is very through in its coverage of the material as compared to other review books. If you're new to the test or are less intense about studying, the Kaplan ACT prep book is also decent.

I actually think a better way to go, rather than getting any of those books, is to get the Real ACT Prep Guide and the ACT Black Book. The Real ACT Prep Guide comes directly from the ACT, so it has the most accurate practice questions. The only issue is that its answer explanations are pretty lame, which is why you should also get the ACT Black Book. The Black Book doesn't have practice problems, but it has a bunch of great strategies and answer explanations for questions in the Real ACT Prep Guide.