Wallace "Wally" West, the fourth Flash, is the fastest Flash.  Up to this moment, however, until Jay or Barry comes back to life in a future story, and DC deems one of them to have found even more speed.  We know Wally is the fastest Flash so far because in the recent "Flash War" he is said to be faster than Barry Allen, the 3rd Flash (and Barry was the fastest Flash until that point).  This makes Wallace not only the fastest Flash but the speediest character in all of DC Comics!