Reverse Flash if they can do something interesting with him. The fight scenes on the TV show can get a little dull, but if they can do something visually interesting than go for it.
A group of the Rogues? I don't want Flash to go against just Captain Cold as he won't see like much of a threat. But Cold with Mirror Master and Weather Wizard? A Jai Courtney cameo would be cool too, but I don't want him to be a main villain. Flash needs to be portrayed as super powerful to separate himself from the TV show where he somehow gets tagged all the time by villains who shouldn't lay a hand on him (Girder anyone?), but Boomerang throwing a couple of trick boomerangs at something would be alright, but then him being like, "stuff this" and grabbing a beer and walking off.
Also, this might get some hate, but how about a non-Flash Villain? I'd love to see a character from maybe the Batman universe crossing over with the Rogues or something? Maybe the Riddler with the Rogues? I think this would bring extra interest to the movie.
Also, I wouldn't want them using all of Flash's main villains because I would like more Flash movies