It should be Barry.
It's the beginning of the Justice League and the overall universe that the DCCU is trying to set up. Barry is the precedent for what the modern Flash legacy has become. His origin can stand on it's own unlike Wally and Bart (whose origins both rely on Barry in one way or another). Plus, he's played a central role in the biggest events of the DC Universe...
Founded the Justice League, generated and powered the Speed Force, discovered the multiverse, saving the multiverse in the first Crisis, outran death and helped end the Final Crisis (alongside Wally to be fair), and made great personal sacrifice to reconstitute reality even if his mistake caused it to fall out of whack. He didn't have to try to fix the problem his attempt to save his mother caused, but he took responsibility and did what he had to even if it caused him the pain of loss.
I wouldn't boycott the movie or anything if it's a different Flash, but I would definitely be very disappointed.
However, I'd rather it be Wally than Bart. Bart is such an outlandish choice, I don't understand why people are suggesting it. He's a great character that I actually like better than Wally in some ways, but he shouldn't be the Flash right out of the gate. And if Wally is the one that they go with, Kyle had better be the Green Lantern.