Is best an abstract noun

Yes, the noun 'best' is a common noun, a Word for the best of anyone or anything.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Dr. Ian Best MD, Pediatrics, Marseilles, IL
• Best Street, Lane Cove, NSW Australia or Best Avenue, Oakland, CA
• Farmers Best Premium White Popcorn
• "The Best Years of Our Lives", 1946 movie with Myrna Loy, Fredric March, Dana Andrews

The Word 'best' is also a verb, an adjective, and an adverb.