Generally a nema23 with 3Nm of holding torque will be suitable for most machines, ok for all 3 axis's, speed and torque are fine and you can't really go wrong.

Really it depends on the machine design and specifics, when looking to select a motor for your x axis, on a moving gantry type machine for instance, you would want to know how much total weight that assembly has, then you could plug those numbers into the stepper motor selection calculator also posted as a sticky.

.Me