Anything from Johns' run is really, really good. Ignition, over halfway through the run, oddly, is a pretty good place to start, as is the actual beginning of the run, or the Rogues trade. So is the Iron Heights one-shot, if you can find it.
Waid's Return of Barry Allen's pretty awesome, as are the rest of the trades from his run.
Of course, going back two months and just reading Flash: Rebirth's probably a pretty good idea too.
I would strongly suggest this: start with the Infinite Earths Crisis (Death of Barry Allen) and try to pick up as many Wally as the Flash comics in order as possible. Here's why; the death of Barry and Wally's involvement thereof will set the tone for the 1st half of the entire Wally series. Although some of the Wally stories may seem a little slow at first, you'll find the first few vestiges of what Mark Waid will use to build on to create the modern Flash myth; Speed force, character motivations, romantic relationships, ect. You will eventualy discover that in many ways Jay, Barry, Wally (yes, even Bart)are very odd characters in contrast with the mainstream DC universe stories. For example, without the Flashes, there would be no "Crisis" arcs. If you are new to the Flash mythos then, as Obi Wan said; "You've taken your first steps to a larger world."