Batman's arch enemy is a homicidal lunatic, Superman has a megalomaniac hell-bent on world domination, and the Flash? The Flash has a hyper-intelligent gorilla. Check out Gorilla Grodd and the monkey fun bunch in these latest screens from DC Universe Online.
I suppose the logical evil opposite to the Flash would be a guy who moved incredibly slowly, but I guess that's not a super power with many practical applications. "Ha ha, Flash! You have overshot me once again!" That doesn't work, and once you've crossed off super slow guy off of your list, the next logical choice is a hyper-intelligent telepathic gorilla.
Of course, there are also villains like Zoom, who manipulates time to go faster than the Flash, and the whole Flash Rogues gang, but if they ever make a new Flash movie, I want Grodd as the major villain. Somewhere in Hollywood there's a warehouse full of gorilla suits just collecting dust. Soon their time will come again.