Hi Arun,
I assume that you have created filters on the report while creating the web service.
If that is case I don't think we can pass the values to filter the ranking itself. Ranking will be derived after the selection of values and at the same time you are trying to filter on the rank values. hence it won't work.
In your case you can only pass the selected values to columns through the prompts in the report and get the required output after filtering based on the rank.
Thanks,
Sampath