As earlier mentioned that all the layers have their importance. Mostly the Network, transprot and application layers are the most important and very useful layers in routing. Here I have provided one URL that will help you to found the applications of different layers and different way as a pictorial representation:
all the layers are important but the most necessory are data link layer and the physical layer because without these two the data can not be transferred but if we miss the session or application layer the transfer is still possible.
'Important' wouldn't be the word I'd use. I'd say layer 3 the network layer would be the most significant layer in optimizing network performance. Google search 'Layer 3 Switches' and you'll find out how they've integrated routing logic into hardware in switches.