matador,
What Nick said (it depends) is bang on and unfortunately the only right answer. Fortunately, what matters is not what the theory predicts but how the helicopter actually performs. Even more fortunate is that the effects of downwash are quite simple to measure, and may already have been for each type you're considering.
Something to consider, above two rotor diameters, ground effect doesn't alter the performance much. Near fires, wind patterns and local air temperatures would probably have a much more significant impact on performance than ground effect would.
This is purely conjecture, but if we were able to eliminate the fuselage effects, then I'd expect that the tandem configuration would have the greatest ground effect advantage while the intermeshing would have the least. My thinking on this is based on the direction of the downwash. With a zero wind condition it's (theoretically) completley vertical for a tandem, tilted slightly laterally for a conventional tail rotor due to the lateral cyclic required to counter the tail rotor side force, and showing relatively tremendous horizontal velocities for the intermeshing configuration due to the fixed lateral tilt of each rotor.
To be fair, that impossible comparison is looking at disk height, and due to the forward flight requirement of the aft pylon to be higher than the forward pylon in a tandem, the disk height of the tandem will typically be higher than that of others.
In the end it's far too complicated to deduce and fairly straight forward to measure.