What motor has the highest torque to speed ratio that can be driven with Arduino.
I realize this question is vague. My apologies. I don't have definite values as
I am experimenting with a lot of unknowns and need to make a test.
Well in a way your question is almost impossible to answer. There is no real limit of how large a motor an arduino board can CONTROL. Where people get in trouble is when they expect an arduino board to be able to supply the power a motor requires, which is very limited.
So in building a motor powered project the order of selection should be:
1. Select the motor that has the required torque/speed that the project requires.
2. Obtain a motor driver module that can support the voltage and maximum current draw of the motor under it's highest load.
3. Obtain a power supply that can supply the needed voltage and current for the driver/motor.
Assuming the motor driver module uses some standard control logic interface, usually it's pretty simple to have the arduino manipulate the motor driver module to the desired functions the project requires.