In no particular order, Jon Jones, Georges St. Pierre, Demetrius Johnson, Jose Aldo, Dominic Cruz, and Fedor Emilianenko. None of them had an obvious weakness for others to exploit.
Aldo, Cruz, and Fedor have looked questionable as of late, but in their primes, their dominance was impressive.
Anderson Silva always appeared to be vulnerable to a good wrestler with sub defense.
Khabib Nurmagomedov *might* be vulnerable to a striker who can stop his takedowns, but no one has managed to test this so far, unless you count Iaquinta, who he outstruck.
Also, while they will be dismissed because WMMA, Amanda Nunes and particularly Valentina Shevchenko are complete fighters.
Aldo, Cruz, and Fedor have looked questionable as of late, but in their primes, their dominance was impressive.
Anderson Silva always appeared to be vulnerable to a good wrestler with sub defense.
Khabib Nurmagomedov *might* be vulnerable to a striker who can stop his takedowns, but no one has managed to test this so far, unless you count Iaquinta, who he outstruck.
Also, while they will be dismissed because WMMA, Amanda Nunes and particularly Valentina Shevchenko are complete fighters.