What is the simplest way to replace a Subaru front power window motor?

Assuming you have tested the window switch and wiring to ensure it is the motor, there really is only one way to replace it, but it is fairly simple.

You can find detailed directions with pictures here.

1) Remove the front door trim. *Caution, Do not apply excessive force to the clips. Otherwise the clip may be broken. Pull up the inner remote cover toward you to remove the upper hook. Pull it down to remove the lower hook. Remove the inner remote cover.

2) Remove the screw, then remove the power window switch assembly and disconnect the harness connector.

3) Remove screw under window switch panel. Then remove the clips of the trim panel using clip remover (or flat-head screw-driver) to remove trim panel

4) Remove the Seal Cover. *Carefully remove sealer, Excessive force will easily break cover.

5) Remove the mirror gusset cover

6) Disconnect the mirror connector. Then loose the screws and remove mirror assembly.

7) Remove the stabilizers and trim hooks. (* Tech Tip, use a marker to trace outline for the position on installation)

8) Remove the rear end of door weather strip and weather strip outer.

9) Move glass to the position shown in the figure, then remove the two nuts from the service holes.

10) Remove the door glass.

11) Loosen the nuts to remove the rear sash.

12) Disconnect the motor connector.

13) Loosen four bolts and two nuts to remove the regulator assembly.

14) Loosen the screws to remove motor assembly.

Installation of the new motor is the reverse of the above.

If it is the same as a Forester, There is a much simpler method than that listed in poisson's answer.

1. Remove door card and seal cover, and unscrew and remove the switch panel.
2. Slacken off the 4 bolts holding the regulator assembly to the door.
3. Unplug the motor from the loom
4. Reach into the door, and undo the three bolts holding the motor in. You will have to do this by feel...
5. Push the mechanism away from the inner skin of the door. This should give enough room to extract the motor. It's quite a tight space, so a job for nimble fingers.
6. Refitting is the reverse of removal! Make sure the regulator is at the top as you refit the motor, as it might sink slightly under its own weight.

Following the answer from Poison Fish, do these:

• Use a short No. 2 or 3 Philips bit on a small rachet. Most other Philips with spin and damage the screw. The chunky square kind like from an impact screwdriver works best.
• Replace the screw with a small M6 10mm head bolt.
• Try tightening the screw first and then loosen as some of these have locktite on them.
• Many times the motor contacts can be cleaned for reuse, unless water has entered and initiated corrosion.
• Used motors from the opposite side in the back seats can be used, though on the newer models they are slightly smaller. Outback motors are almost identical, but slightly bigger and may be reversed in symmetry.