Question

Who perfected the incandescent lightbulb?

Answer 1

William D. Coolidge, 1911, if "perfection" means a major decrease in further enhancements.

Explanation:

You may want to clarify invention, and whether even now it is "perfected". Many claim Thomas A. Edison, but even as an "inventor" that is disputed.

While people often credit Thomas Edison with inventing the light bulb, it was actually invented 60 years earlier, in 1820, when a man named Warren de la Rue passed an electrical current through a platinum coil inside an airless tube and produced light. Thus the first incandescent light was born.
http://light-bulbs.us/history.htm

Many different people at different times and places improved the technology. Edison may have first made the practical vacuum to light bulbs, but that was far from "perfecting" them.
http://edisontechcenter.org/incandescent.html

William D. Coolidge had been working with tungsten which proved to be a superior material for a long lasting lightbulb over any other material to date. Coolidge figured out how to heat tungsten and draw it out through heated dies of decreasing diameter.

The result of his work was a workable, bendable (ductile) wire that was high strength and made a great filament material. The new material was used in bulbs in 1911 and this is still used today. See our inventors section below for more advancements in the incandescent bulb.
http://edisontechcenter.org/incandescent.html