The 27-year-old has been a regular fixture in the Partenopei's defence since he arrived from Genk in 2014.
Koulibaly made the Serie A Team of the Season in the 2015-16 and 2016-17 seasons and also earned himself a place in the Caf Team of the Year in 2016 after his fine defensive showings that helped Napoli across the two campaigns.
The Senegalese centre-back who withdrew from international duty due to an injury last Friday has played in every of Napoli's matches this season.
"Koulibaly is currently the best defender in the world," Stendardo told Radio Kiss Kiss per Football Italia.
"The only one who gets close to him is Sergio Ramos, who is in my view physically inferior to the Napoli man, who is a perfect athlete.
"He improved so much over the three years under Maurizio Sarri when it comes to playing with the ball at his feet, becoming very good at building moves from the back."Koulibaly was heavily linked with a move to the Premier League in the summer before signing a new five-year deal with the Naples outfit in September.