Never use the rings with the plastic funnel. Some folks like the waxless seals, others prefer the wax rings. Either seem to work fine. Since your flange is inset into the floor, I think the thick ring would be the wax ring of choice. Another tip that might be helpful. Take some emery cloth or just plain sandpaper and sand around the tank bolt holes in the bottom of the tank. Sometimes there may be just a bit of roughness than can prevent a good seal. Just a quick sanding is all it takes.