excellent vs first-class

excellent and first-class both are adjectives.

Word	Noun	Adjective	Verb	Adverb
excellent	No	Yes	No	No
first-class	No	Yes	No	No

As adjectives, excellent and first-class are synonyms defined as:

Other synonyms of excellent include fantabulous, mean, ripping, splendid.

excellent (adjective)	first-class (adjective)
very good; of the highest quality	very good; of the highest quality