Alright, here's what I think.
Voldemort has some good advantages. He's at least one tier higher than Flagg with some of his raw attacks, has enough spells to match the Walkin' Dude's versatility, and is also one of the few characters I know of in Tier 8 who can legitimately do something against Flagg's absurd range. He'll be shielded from Flagg's mind reading to an extent, and he's got experience fighting other wizards. (Which is implied to be the case with Flagg too, but not outright shown)
Problem is, Flagg has counters for everything good that he can do. Every spell that isn't Fiendfyre-level or higher will just bounce off Flagg's barrier once he has it up, and Fiendfyre itself can be countered thanks to Flagg's teleporting range, which is at least as good as Tom's if not much better. (He teleported away from that nuke I mentioned some time ago and was next seen on a remote beach where the natives didn't even speak English.) And while Voldemort has his mind reading, Flagg actually showed in one of his most humiliating moments that he can "feel" when people are reading his mind, so that won't be much of an advantage. A direct mental battle is also a bad idea for Tom. Very bad.
Beyond that, Flagg's magic and psychic powers are a lot scarier than Voldemort's. In terms of "ending the fight", Flagg has more options, better options and better range on a lot of those options. And as far as Voldemort's hax defenses, they don't cover enough of what his opponent can do here, and he's never dealt with any mind control, psychic attacks or illusions as powerful as what Flagg has shown.
In the end, it just boils down to the Dark Lord being outclassed. Not a stomp, but decisive. Voting Flagg.
