second best vs competitor

second best and competitor both are nouns.

Word	Noun	Adjective	Verb	Adverb
second best	Yes	No	No	No
competitor	Yes	No	No	No

As nouns, competitor is a hypernym of second best; that is, competitor is a word with a broader meaning than second best:

Other hypernyms of second best include challenger, competition, contender, rival.

second best (noun)	competitor (noun)
the competitor who finishes second	the contestant you hope to defeat