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.
That's exactly what I was thinking. The 1911 barrel floats all over the place. If you put a tight target bushing on it and make the rear lockup super tight, then you lose reliability. Then there is slide fit. A custom gunsmith that takes a lot of extra time and effort can make them shoot like target guns but your basic stock 1911 isn't all that accurate. Actually when I was growing up, lots of oldtimers would claim you couldn't hit a barn with one. From inside the barn.
Out of my non-custom semi autos... I would say my HK P7 is the most accurate. Too bad the barrel is so short. If they had made a version with a longer barrel and longer sight radius, it might have been a world beater target pistol.