All would make for nice looking backgrounds, depending on tank and fish type. Definetly blue for salt and black for a planted tank. the gradient will give you a deeper visual effect then the solid.
Edit: Apistogramma Master's tank looks quite nice with the black. so not always blue for salt
I voted the Blue gradient. I'd start darker at the bottom, midnight blue, and work up to navy. or black. or just do the whole thing midnight blue. that's what I'm planning for my next tank.