As far as the director goes, I'd prefer Zemeckis over Vaughn or Raimi at this point.
The Flash movie is going to require a heavy amount of near-constant special effects, which is undoubtedly something Zemeckis has extensive experience with and excels at. Along with those skills, he certainly has been know to have a unique visual flair with SFX and his films overall. I don't want to see half-assed, completely generic speedster effects (looking at you, Quicksilver in AOU). I definitely feel like Zemeckis would work hard in this field and strive to build on what Snyder did with The Flash SFX in Justice League, in order to create something that is visually breathtaking and nearly photo-realistic.
But beyond that, his track record has shown that he knows how to craft compelling stories that are sometimes delicately and uniquely structured. He takes risks and hasn't only limited himself to one or two genres of film throughout his career. Look at the stark differences between two of his greatest films -- Forrest Gump (which has the kind of blend between comedy/drama that The Flash needs) and Cast Away. (Speaking of Cast Away, the opening plane-crash scene is an incredible action sequence. Just saw it again on TV last week.)
And of course, as everyone knows, the concept of "time" in essential to a character like The Flash. Time -- whether it be time traveling or the passage of time -- also happens to be something that Zemeckis has show interest in throughout his career, so it does seem like The Flash is the project that may fall into his wheelhouse.
Now, I'm not suggesting that Vaughn and Raimi lack the talent or expertise to make this film great at the end of the day, but I do feel that Zemeckis is a little more of an "outside the box" pick and that's something I like. With the other two, it feels like they might be more "obvious" and safe picks because they each made a couple of successful films featuring younger, lighthearted, and jokey superhero characters, where as Zemeckis has not yet directed a superhero/CBM film.