Question

What is the biggest planet in the Milky Way?

Answer 1

It is difficult to distinguish between a planet and a brown dwarf but, the biggest known planet in the milkyway could possibly be
HD 100546 b

Explanation:

The definition between a giant exoplanet and a brown dwarf is pretty vague and hence the planetary status of most such large bodies discovered remains to be confirmed.
but according to wikipedia,
the current recordholder for the largest known exoplanet in the milkyway is HD 100546 b.

For further reading on other massive exo-planets:
List of largest exoplanets
For further reading on HD 100546 b :
HD 100546