So the archangel Ludociel was given the grace "flash" by the Supreme Deity. It's said he can move almost instantly. Does this make him the fastest character in the series? Or do characters like Meliodas or Ban who have been shown to be very fast compete with his speed?
We do not know the upper limits of Flash, neither do we know how it fares against other quick characters.
There have been no satisfying points of comparison between Ludociel and other extremely quick characters. The wiki only notes that "[it] allows Ludociel to move at immense speeds which appears as a form of teleportation." which is consistent with Flash's depiction within the series. Both Derrierie and Escanor, two experienced fighters ranking among the strongest characters in the series, were completely unable to follow Ludociel's movements.
Flash, generally speaking, has been scarcely used and is thus hard to describe and quantify. If we try to understand its inner workings by comparing it to other angelic Graces, we can infer its strength directly stems from the natural element of light (it could be lightning thematically, but we have seen numerous weaker characters reacting to and dodging lightning). Both Ocean (from Sariel) and Tornado (from Tarmiel) have demonstrated the ability to create a pocket universe almost exclusively containing their own element (manifesting as a windy ocean when they combine their abilities).
The best explanation we can try to offer for Flash is that it is a light-based Grace allowing Ludociel to literally move at the speed of light, thus explaining his extreme unmatched speed. If this theory is true, it is very unlikely other characters could possibly match him in a speed contest.