Most contributing resonance structure of phosphoric acid
Both structures are not the main contributing structures. In fact, the first structure is outright wrong and should be marked as an error as the electrons of phosphorus do not add up and the non-protonated oxygen is implied to be a radical.
However, the correct most contributing structure can be derived from the first structure simply by adding formal charges: a formal positive charge on phosphorus and a formal negative charge on the non-protonated oxygen. This structure satisfies the stability conditions well:

All main group elements have octets (dublets for hydrogen)
The second proposal violates this most important principle

The number of formal charges is minimalised
We cannot go any lower without violating the more important first point

The type of formal charges matches the electronegativity difference
And also we cannot distribute them differently

The distance between formal charges is minimal
But again we have no choice.