A few years ago I would have given PARC the thumbs up, but. . . . . recent experiences by colleagues would have to suggest they are not too good anymore.
Rishworth had a fairly bad reputation on here, but on my current contract (which is through the Swedish office) my Rishworth colleagues are notably more satisfied with their agency than the PARC ones (for the moment )
I have however heard bad vibes about Rishworth NZ, & indeed if you search on here you will find negative comments about all of them (no exceptions I can think of)
At the end of the day, a lot depends on your expectations. None of them will ever fight your corner against a company, even if you are an integral part of the deal, the company is the paymaster.
If the company defaults on the payment , you are normally stuffed, as few (if any) take payment up front in advance to cover this scenario (one I knew of & thought always did, let a bunch of guys down when a UK ACMI company went tits up, so even the "safe" ones aren't always)
As always it is best to speak to guys actually on a contract you are interested in, to hear the latest on who appears to be doing the best job if several agencies are offering the gig.