A gun with a fixed barrel, with integral chamber, SHOULD be the most accurate, as you don't have to worry about tight/consistent lock-up, or cylinder alignment, or any of the stuff that can affect most centerfire autos and revolvers. Unfortunately, that would exclude almost all modern repeating handguns.
I have also heard that the SIG P210 is the most accurate "service pistol". Not as much time and energy is expended on improving the accuracy of any gun as is spent on the 1911, and that's why there are extremely accurate guns of that type.
Les Baer offers a "1.5-inch accuracy guarantee" on some of their 1911 models, and I don't know that there is a maker of any other type of semi-auto that "guarantees" that level of accuracy. Of course, the guarantee is that the gun HAS shot a 1.5" group at 50yds, not that it WILL do so in your hands.