Oda has made the difference between "greatest" and "strongest"
If he wants to say greatest, he would use "ichi no" (like he did in chapter 700)
If he wants to say strongest, he would use "saikyou" (like he did in all other instances when describing Mihawk)
Its like calling Whitebeard (sekai saiykou no otoko), the world greatest man which doesn't make sense