Whoever he fights next.
If Khabib loses, he'll go from the most feared Lightweight in the world to an overrated can according to Sherdog Gospel. Type of things you'll hear are that he never beat anybody in the top 5 (and only once beat somebody that was in the top 10), that he became champion by beating an unranked fighter and that he couldn't even finish said fighter, that he was a good enough style match up for McGregor despite his elite ground game simply because his stand up was so shitty, etc...