Most advantageous decoupling for multiple voltages
Could be that I have a completely wrong assumption on how decoupling capacitors work, but here goes:

You have to analyze the current loops and place decoupling capacitors in the loop to help reduce the size of the loop.

So let's assume you have some ICs working on the top rail and some working on the bottom rail. So you want decoupling on both rails. This approach would rule out the middle suggestion, there is no decoupling on the bottom rail.

The left approach helps if you have currents which will flow from the top rail to the bottom rail, but I guess most of the ICs will have current loops from their supply to ground. So the left solution will provide decoupling for the top rail and the bottom rail, but the top rail sees a series connection of capacitors to ground which will result in a lower capacity and thus might reduce the effectiveness of the decoupling.

So I'd vote for the right approach because that provides decoupling on both rails with the actual capacity of the capacitors for both rails.

I'd also say that an electrolytic cap alone will not provide very good decoupling as they have higher ESR and ESL values, which is not what you want in decoupling, you want to provide a very low impedance path for the noise to go. So placing a ceramic cap (or several with different capacities for different noise frequencies) as close as possible to the IC is usually a good approach. Even the capacity between the power plane and the ground plane around an IC can act as a decoupling capacitor for very high frequencies if the connection is done in a low impedance way.