first-class vs excellent

first-class and excellent both are adjectives.

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

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

Other synonyms of first-class include fantabulous, mean, ripping, splendid.

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