Probably would have been better to narrow the field a bit.
Say "the most accurate autoloader that has been a military issue sidearm" for example.
In that respect the 1911, when accurized for competition, would be hard to beat.
The SIG 210 does have a sterling reputation as a target pistol, but not that many have heard of these.
Only reason I heard of them was comparing an old French .32 Long auto I once had to the SIG because there was a strong resemblence in grip and profile. That French pistol had a great grip and was a natural pointer, not that accurate with .32 ACP, and couldn't find the French ammo back then or I'd still have it.
A shooting test of late 19th century military handguns found the most accurate was the Colt .36 Revolver converted to .38 Navy caliber. They even found a full unopened box of the original ammo to test it with. A very expensive shooting session to say the least.
I have found my .36 replica to be extremely accurate and easy to shoot, the extra long barrel and sight radius is most likely why it scores so high.