what is the best receiver for s500 quad qx7

you can go for RX8R or X8R, they are good receivers.

you can use any frsky receiver with QX7.
you can also use UHF module and use UHF receiver also.

frsky L9R:-

link:-
http://www.rcproduct.in/product/frsky-long-range-receiver-non-telemetry-l9r-912ch-s-bus-accst/http://www.rcproduct.in/product/frsky-long-range-receiver-non-telemetry-l9r-912ch-s-bus-accst/

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Frsky X8R

Link:-
http://www.rcproduct.in/product/x8r-816ch-frsky-s-bus-accst-telemetry-receiver-wsmart-port/

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Frsky V8fr

Link:-
http://www.rcproduct.in/product/frsky-2-4ghz-8ch-receiver-hv-v8fr-ii/

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FrSky R9M/R9

Link:-
http://www.rcproduct.in/product/frsky-r9mr9-long-range-radio-system/

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These all above receiver can work with Qx7.

many parameters govern effectiveness of receivers.

check sensitivity at priority.

prefer if diversity integrated / offered.