I'm a bit unclear about how to think of the energy transfer in a transformer and how it would be optimized. On the one hand, if the primary winding of a transformer is more resistive than the rest of the equivalent circuit (the power source's internal resistance and any connecting wires), most of the voltage drop would be across the transformer, but on the other hand, if the resistance of the primary winding was lower than the connection to the transformer, the I^2*R losses of the primary winding would be lower. Basically my question is, assuming the entire primary side of a transformer has some given and constant resistance, would the transformer be more efficient if the primary winding had the highest possible portion of the resistance or the lowest portion of the resistance?
I think you are on the wrong track with this question. The primary winding should have ideally zero resistance but it needs an impedance to prevent it shorting out the AC power applied. So, it has inductance and, for a typical power transformer that might be in the realm of 10 henries for 50/60 Hz applications.
The transformer equivalent circuit is shown below: -
With the secondary unloaded (not connected to a load) the primary impedance is due to \$X_M\$. Core losses and other resistive losses are shown as resistors. Leakage inductances are shown as \$X_P\$ and \$X_S\$.