Oh boy... my favorite? Lathe?
Although the most commonly used "power tool" would easily be a ultrasonic cleaner (and the most useful too), followed by maybe the pivot polishing machine? The lathe is mainly used to make brass/bronze bearings for clocks in case a fitting one is not already available (they can be purchased in sets, ready to use). But the lathe is certainly the most fun to work with.
The electric bench drill also comes in handy very often. And I guess if I consider an ultrasonic cleaner a power tool, then so would be a timegrapher.
If you want to consider electric light to be a "power tool", including flashlights, then that would take first place.
...But Ill tell you about another fun power tool, which even beats the lathe. Pressure washer!