I would consider Zoom to be Barry's archenemy due to them having the same power set and the events going on in the Flash: Rebirth series. Other than that, I can't think of any villians that would really be an archenemy.
But Zoom didn't appear until towards the end of Barry Allen's career. I don't deny the damage he caused at the end, but how can such a late-comer be considered his arch-villain? And he only fought Wally West once. That leaves the rest of the Rogues, and no one of them can be considered a greater villain that the other, really.
He's till gotta have a nemesis though. Spiderman has a rogues gallery too, but his arch nemesis is Green Goblin. Batman has a gallery too, but he has Joker. I'm inclined to say Zoom, but I agree with Leafman in that it depends which Flash.
Flash doesn't really have one archnemesis like Lex Luthor to Superman or Joker to Batman. He has a Rogues gallery. A list of villains that regularly make a run at him, and often times together. These are the guys:
I believe his archenemy is Professor Zoom a.k.a Reverse Flash. I think it depends on which Flash you're talking about though, Zoom is more closely associated with Barry than Wally.
Well he is the Rogues' leader when they all team-up together, so I guess it would go to him before the rest.
-Nate-
-Nate-
-Captain Cold
-Mirror Master
-Weather Wizard
-Heatwave
-Captain Boomerang
-Reverse-Flash
-The Piper
-Gorilla Grodd
-Nate-