There really isn't one. Aldo had a great run, longevity, was a breakthrough star that put the weight on the map way more than the previous generation of guys like Pequeno, Curran, Menjivar, and Hansen, and beat excellent competition like Edgar, Faber, Brown, Mendes, KZ, Florian, Cub, and Moicano.
Volk beat two other FW goat contenders in Aldo and Holloway x3 (one very close fight), plus Mendes, Yair, Ortega, Lopes, Elkins, and KZ (though an old KZ) to boot.
Then you have Holloway who beat Aldo twice, plus Edgar, Yair, Ortega, KZ (an old KZ though), Pettis, Oliveira, and Cub.
Volk beating the other two on this list is huge for his legacy, and he beat a prime Holloway, though Aldo was already past his prime (just couldn't be helped as their primes didn't overlap) so that (I believe) makes this difficult.
Too bad McGregor left the division early as he had a pretty nice run going and could have built a legacy to contend on this list since he had wins over Aldo, Mendes, Porier, and a young Holloway in a short time. Wish he would have dropped back down after beating Alvarez. I hope Topuria doesn't make the same mistakes.