The Flash vs Every DC/Marvel character with these exceptions
they can't be people who can fly (ie Superman), use magic (ie Dr.Strange), mind abilities (ie Xavier), or energy projections (ie Magneto)
I know that's a lot of limitations, but I didn't know how better to put it
but that still leaves alot on the list (hulk, colossus, the thing, venom, etc)
I give it to the Flash. I just saw a Justice League Unlimited episode "Divided we Fall" where the Flash beat the living Brainiac out of Luthor, who was in the episode responsible for effortlessly knocking out the entire JL in a matter of seconds
The Flash, when utilizing his potential, is uber-powerful