None of the above. He seemed closest to Wonder Woman than anyone in the JL series. If we're talking about the comic versions of the characters then it's between Batman and Hal.

@avenging_x_bolt: I meant the roster was based of the show not the friendship(thats based of comics sorry for not being descriptive)

@cyborgfanatic: no prob, thanks for clarifying.

Probably Barry, apparently they used to talk to each other about being Policemen, Green Arrow can't even really talk to him unless J'onn shapeshifts into a human and Wally is grossed out when J'onn changes into his native form. Superman could be a candidate as J'onn has known him since Clark was a kid and even went to Christmas with him. MMH doesn't really care for John due to his role in the destruction of Xanshi