stellar vs leading

stellar is not a noun while leading is a noun.

Word	Noun	Adjective	Verb	Adverb
stellar	No	Yes	No	No
leading	Yes	Yes	No	No

As adjectives, stellar and leading are synonyms defined as:

Other synonyms of stellar include prima, star, starring.

stellar (noun)	leading (noun)
	thin strip of metal used to separate lines of type in printing
	the activity of leading

stellar (adjective)	leading (adjective)
being or relating to or resembling or emanating from stars	going or proceeding or going in advance; showing the way
indicating the most important performer or role	greatest in importance or degree or significance or achievement
	having the leading position or higher score in a contest
	indicating the most important performer or role