What is the best general purpose variable resistor in a battery discharge circuit (10V < Vbat < 50V)?
Don't try and mimic a resistor. Design a voltage controlled constant current sink. It will be much easier and more exact. Here is a brief idea: -



If you choose \$R_R\$ to be 1 ohm, with 5 volts at \$V_{IN}\$, the current drawn from the battery will be 5 amps. However, \$R_R\$ will dissipate 25 watts so it can be scaled down using a 0.1 ohm resistor.

However, the circuit is linear and either the resistor gets warm or the transistor gets warm or both. It will need significant heat-sinking.