Now this article was incredibly tough to write since the brilliant CW TV show, but without further ado, here we go. (I have decided to not put Eobard Thawne on this list, simply because he cannot be topped.)
Captain Cold/The Rogues
Although they would be going up against Wentworth Miller's amazing interpretation on the character I feel Cold could be awesome. However, to avoid a Spider-Man 3 situation, I would like to see the Rogues as established characters, so we wouldn't need an origin story.
If the filmmakers wanted to differentiate themselves from the TV show by making Ezra Miller play Wally West, then the villain should be Hunter Zolomon A.K.A. Zoom. Zolomon's a sympathetic villain who wants to make the Flash's life a living H*ll.
Mirror Master (McCulloch)
An extremely tragic villain who shot his father by mistake and returned home to see that his mother had killed herself. However, the main reason I want to see him is the fact that there would be an amazing fight scene in the Mirror Dimension.