I think the piston is more reliable, but that doesn't make it the best. Every type of gun has advantages and disadvantages compared to other types. A bolt action is generally more reliable and accurate than any DI or piston semi auto could ever hope to be. But DI and piston systems offer advantages over bolt actions that make both of them better suited for many situations despite not being as reliable or accurate.
In terms of just reliability and nothing else, I think the piston wins hands down between DI and piston semi autos. That doesn't make a piston system better than DI just like it doesn't make a bolt better than both systems. A DI still has advantages in other areas that make it better suited for many situations.
As far as which is the "best" overall system, well that depends on the situation and the individual. A bolt action still seems to work pretty well for snipers and hunters. A dirt poor farmer in a war torn third world country is probably going to want a piston system? Solders and Marines seem to be doing ok with the DI system. It all comes down to the situation.
At least that's my .02