First, let's see the Pokemon with their HP stat in descending order:
/ds hp desc:
Eternatus-Eternamax, Blissey, Chansey, Guzzlord, Regidrago, Wobbuffet, Wailord, Snorlax, Drifblim, Giratina, Eternatus
Out of these, Eternamax-Eternatus and normal Eternatus can't Dynamax. Blissey and Chansey were there before DLC 1, Guzzlord and Regidrago as well. So, excluding these Pokemon, Wobbuffet has the highest HP stat among all that can Dynamax.
Wobbuffet's Base HP is 190. At level 100 with 252 EVs and 31 IVs, it hits 584 HP stat. When Dynamaxed at Dynamax Level 10, it would be 584 X 2, which is equal to 1168.
Therefore, Wobbuffet has the highest HP when it Dynamaxes, and before DLC 1 or Isle of Armor, with its HP stat reaching 1168 when Dynamaxed.
Hope this helps!