vs.

Gyarados

I got a couple ways this could pan out but lets lay out the facts first,

The Flash: All incarnations of the Flash can move, think, react at superhuman speeds, vibrate so fast that they can walk through walls, travel through time and can also lend and borrow speed. His weakness pretty much anything he cant get away from.
Gyarados: A Dragon-type pokemon with high attack “stat” Gyarados is infamously known for its fierce temper and and destructive tendencies. Once it has worked itself into a frenzy, it will not calm until everything around it is destroyed. His weakness would be electricity

First Scenario: Battle One- You have to agitate Gyarados, he doesnt go looking for anyone he needs the water surrounding him for attacks so The Flash would have to come in firs- oh hes already there ok nevermind. Now if I was the Flash I would immediately see water and begin to run in circles until I created a cyclone locking in Gyarados. Gyarados would go into a rage and begin to glow, faster and faster the Flash picks up speed FWOOOOOOOOSHHHHHH hyperbeam. The trail the Flash carried would catch on fire incinerating his body the Flash freaks out and dives into the water and Gyarados uses “Surf” to drownd him.

Second Scenario: Battle Two-  This isnt exactly fair I meen Gyarados is a fucking dragon that can shoot beems and control water while the Flash can run fast, and punch fast, and yea fast. So lets say Gyarados was caught off guard by a pokeball. While Gyarados struggles too get out the ball Flash grabs it and runs straight to a nearby power plant and as Gyarados breaks free the Flash tosses the ball into one of those “electric zap like” towers Gyarados would become entangled in the wires and go bat shit crazy sendng a bazillion volts of electricity through his Magikarp ass.

So everything said I give the nod to Gyarados. The Flash is too wack to really be a hero hes only “super” because of his ability of speed but once somthing stops him hes just a squishy regular person. They should really let him control lightning or somthing, but what do I know.

Some more epic battles for your enjoyment:
uno- The Hood Internet- Grizzly Bear vs. Dead Prez
dos- Lil Wayne vs. Sleigh Bells- Fireman On The Ground
tres- Blink 182 vs. Lil Wayne & Birdman vs. Train- Stuntin 182
source: Pigeons and Planes,Too Much Happiness,My Old KYHome