Usually, when this discussion arises, people say it's between Barry and Wally. Honestly, it depends on which time period you're talking about. At first, Barry was the fastest. At the same time, just about every Flash (only considering Barry, Jay, Wally, and Bart) has been the fastest at one point in time. Jay was the fastest in his prime, when he was the only Flash, and part of the Justice Society. Then Barry came along. He was the fastest while he was the Flash, and even when his nephew Wally came along.

None of this is based on Young Justice, by the way, since if it was it might not be very accurate.

Wally was the fastest Flash when Barry died (39 years), though there was that crisis in which Wally wouldn't run anymore. I'm pretty sure he gave up his costume to Jay and then told him he could be the fastest again.

Then there was Bart.

One important thing to remember is that, well, Bart is from the future. The Bart I'm more used to is the older one, the one you might find in the old Teen Titans comics. The Bart others might be more familiar with is the Bart in Teen Titans New 52 and the Flash New 52.

In New 52, Bart came was transported to the future because he was under some sort of witness protection or something. Bart's name wasn't Bart, it was Barr Tor, and in New 52, Bart isn't related to Barry at all. The people that sent him to the past gave him an artificial memory, and a belief that his name was Bart Allen. So, of course, Barry wants to know who this new speedster is whose also adopted his name.

That's why I hate New 52. Brushing New 52 aside, Bart was the fastest Flash when Barry was gone (again) and he (somehow) went in the speedforce as a teen and came out man.