The most comfortable economy seat I found was on Lufthansa 747s after they redesigned the interiors in the late 1990s. (I believe that design overhaul was done by Frog Design here in the SF Bay Area.) The new seats had a bit more support (depth) under the thighs and fairly firm seat cushions, as well as the adjustable, winged headrests (which many pax to this day seem to be unaware of).