All of them, and none of them.
Accuracy tends to be a huge collection of factors, and pretty much any pistol can be made more than accurate enough. If you're sweating over the millimetres, then the only advice I can give for maximum accuracy is to find something that has a good quality aftermarket TDC hop unit, an outer barrel that doesn't move around too much, a good hop bucking, and probably a 6.05 or 6.08 inner barrel (as a 6.23 widebore would likely see a significant dip in power on a GBB pistol that would actually hurt the accuracy). If it was a target pistol, I'd make sure the hop setup can handle it, then run .48s through it for the most consistency in the BB flight path, but I wouldn't do that in game because it's my opinion that people who run .48s in pistols are being a dick, and the golden rule of airsoft is "don't be a dick". In game I run .32s in my pistols for the trade off between performance and etiquette (and cost!)