Here we go broskis. We all know MM is the unsung hero of the JL possesing the greatest abilities and power. But then we have Barry Allen, who is basically OP with his speed techniques. Now it's debatable which one of these two is the best, so I'll let everyone here decide. (1). Pre-Flashpoint for each. None of that New 52 stuff. All feats count, as long as they arn't completely PIS-Redonculous. (2). In character. (3). Morals are on. (4). Fight takes place on an abandoned Earth. Who wins?
this fight will end in less than a second, the problem is who's gonna finish the other in less than a second, if Barry with his speed get fast enough to escape MM's telepathy he wins with a super speed punch, if MM manage to paralyze Barry before he go faster, he wins.
Barry doesn't even do half the things Wally has done. And I believe it was Wally who forced J'onn to be tangible, and then he got mind wrecked. With morals on, he's not going to deal with BOTH intangibility and telepathy. It's unlikely Barry would do any better in this scenario.