Answer

The Critique of Pure Reason is his main and most important book. But reading it is a very difficult task. In my opinion, most Kant experts also haven’t read it in full (and neither have I). It’s better to start by reading about Kant (Bergman’s book and other books), and then, if you want, you can focus on what interests you.
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lodrikan:
Is the Critique of Judgment more readable?
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Rabbi:

Yes. But it’s still not easy, and I’m not sure it’s worth the effort.
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lodrikan:
A lot harder than Groundwork?
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Rabbi:
Harder. But instead of all this back-and-forth, just start and see. The books are accessible, and you can read and see what’s difficult and what’s worthwhile. I suggested what seems right to me, but of course everyone has their own taste and interests.

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lodrikan:
Sorry for the bother, just one last question that I couldn’t find an answer to.
Do the books depend on one another? (That is, if I haven’t read the Critique of Pure Reason, will I understand the Critique of Judgment?)
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Rabbi:
There’s no significant dependence between them (though there are connections). Understanding one doesn’t depend on having read the other.