I think it's Ubereem
Mousasi never fought Yoel Romero.I think it's Overeem. Mousasi is still young and can steal it from overeem in the future, but as from now he hasn't done much in mma other than beating chris weidman and Yoel Romero.
Mousasi has a hell of a resume. Alistair is a close second, IMO.
Sorry, but he doesn't come close to Overeem...
Overeem
Brock lesnar
Fabricio Werdum 1
Fabricio Werdum 2
Juniour Dos Santos
Sergey Kharitonov
Igor Vovchanchyn
Andrei Arlovski
Vitor Belfort 1
Vitor Belfrot 2
Frank Mir
Mark Hunt 1
Mark Hunt 2
Meanwhile Mousasi has
Mark Hunt
Jacare
Chris Weidman
Lombard