TPG's for Currency - What Are the Acceptable Ones?

I've got PLENTY of experience with coins and their TPG's.
But now I'm looking at starting a currency type set (small-size $1).
I see some "slabbed".
What are the acceptable TPG's for currency?

 

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the top tier TPG's are PMG and PCGS. i would probably stay away from most others. PMG is part of NGC and you know PCGS

 

I too, agree with Urbanchemist. in that specific order as well.

 

Thanks.
The only one I was curious about was CGA.
Okay, it's out.

 

I'm wondering if paper currency will ever have a CAC type decal attached to the holder.....in other words, a grading company grading a grading company......like on the coin side.