Most accurate pistol? Maybe the Mosin Nagant Obrez Pistol (not the revolver)
7.62x54r Bolt action, scoped.
One of the main problems with pistols and accuracy is that the bullets generally suck for accurate fire. Low velocity+ big bore = bad trajectory and more prone to wind influence and that's on top of having bad Ballistic Coefficients, which further exacerbates the poor trajectory and wind influence problems.
The .44 magnum is touted as accurate at 4-5 inches at 50yrds. Pistol accuracy is largely dependent on the shooter. Your heartbeat could make the sights quiver enough to throw a 50yrd shot off by more than half a foot because the weapon is so short.