Thanks for your suggestion , as you told i added all my polygon to one layer and added that layer before my feature layer so that my feature layer is always on top now onClick is working fine.
But map looks little ugly it would be great if ploygon sit on feature layer , can you please explain me about query event .When i click on the map (when polygon layer is on top of my feature layer ) how will get the graphic object of feature layer since i am dependent of graphic object attribute (CNTRY_NAME).