That's up for debate. Some might say he's not even the best CDM in his own team
Mascherano is both the best CDM in the world, and not the best cdm in his own team.
"wut zach, you contradict yourself again"
Wel no, settle down and lemme explain.
When he can play as an actual cdm, he's without competition the best in the world. He does that at Argentina, and in big games is always man of the match. He was legendary good in the previous coppa america for example @CrimsonianKing .
At barcelona, they dont use a cdm. They use an excellent passer that holds the midfield. Because of the special pressing style barcelona uses, it works unless you go against a well drilled unit. then their midfield is shit, but they offset that by having insane forwards, so teams sacrifice so much,barca's midfield keeps hanging in there when not in possession.
Its a nice example of defending trough exceptional attackers in the proximitry. Much like you dont have your fullback press deep cause cuadrado will rape its stamina before you can say "peasant".
Because of this, he's the best cdm in the world, but not at barcelona since they use an onorthodox style without an actual cdm.