In terms of accuracy, the ranking is
1) a ruler
2) the Shimano version
3) all the others
"All the others" measure the distance between a roller pushed one way along the chain, and a roller pushed the other way, and thus include the amount of play between the roller and the inner link bushing (or equivalent). There's no reason to assume that this play is either consistent between differeent chain manufacturers, or constant as the chain wears. I've certainly come across people grumbling that a brand new chain shows up as half worn out.
The Shimano checker has 3 prongs so that the two rollers it is measuring between are diplaced in the same direction. It's pretty expensive though, and a ruler is much cheaper.