Top 10 at minimum. #5 at most. Werdum was an extremely strong win, and beating Overeem was pretty good too, but the division is just weak. His win against JDS was years after JDS was relevant. Ngannou is definitely a good win, but at the same time who had Ngannou really defeated? A gust of wind knocks out Arlovski and Overeem, they were tailor made for him. He fought one decent wrestler in Blaydes, but Blaydes is pretty much a nobody with no big wins.
I dunno, there just doesn't seem to much "oomph" in his wins, other than the win against Werdum. Look at other champs; you see Whitaker and it's like, holy shit he just demolished Jacare and took out Romero, those are gigantic wins. Or DC taking Rumble out like it was nothing and having a sick fight against Gustafsson; even with the loss to Jones, it still looks better than Stipe's wins. You look at his last 6 wins and it just feels like he beat up a bunch of old out of their prime dudes and an untested prospect who secured one strong win against an opponent he matched perfectly with stylistically, even if they are the top of the division. The division is in a serious slump and as much as I really like Stipe, the sad truth is that it does cheapen his wins and his record.