Which H atom in vitamin C (ascorbic acid) is most acidic?

The most acidic proton gives more resonance stabilization through the delocalization of stable carbanion.

A carbanion (bearing a negative charge) formed by the abstraction of an acidic proton transfers its electrons to the adjacent empty orbital (delocalization) and forms resonance structures.

The more the resonance structures, the more is the stability of the compound.

Therefore, the compound readily releases its proton to get stabilized by the resonance.

The OH bonded with the alkene undergoes delocalization and forms three resonance stabilized structures through carbanion's formation after the proton's abstraction.

Therefore, this OH group has the most acidic hydrogen.

Formation of resonance structures

The other three -OH groups show their resonance stabilized structures as

Formation of resonance structures