It's obviously unanimous that UWorld is not only the most important but also the one to finish on.
However, since the final month's focus should always be FA + UWorld, no questions asked, since USMLE Rx is essentially a hardcore reinforcer of FA, I would leave out Kaplan altogether. No one would deny that Kaplan could obviously help, but if it's Rx or Kaplan, in the final month, given that you've already finished UWorld, Rx is the one to go with, hands down. If you haven't finished UWorld, focus on that alone until T-minus 2weeks. Then in the final two weeks, just FA and maybe some NBMEs, but not Rx. Only do Rx >2 weeks out if you've finished UWorld. Enough said.
I'm almost 85% finished with Rx at this point, and even though I still know my knowledge of FA needs tweaking, without it, there's no way I would have as deep of an understanding or recognition of the subtle details in the book as I do now.