Answer 1

Some Pokémon-ex also have a large amount of HP; cards previously had a limit of 120 HP, but Pokémon-ex ignored this rule and had up to 200 HP. Before the Diamond & Pearl Great Encounters expansion, Wailord ex had the title of the most HP: 200. This was later tied with Wailord in the Great Encounters series and Wailord in the Dragon Blast series.

So, Wailord is the Pokemon with the Highest HP in TCG.
Source: Bulbapedia

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As for attack-wise, it is Rushing Flames, with a max of 4640. However, any card that says Flip a coin and do x damage for the number of y times you get heads/tails can do an infinite amount of damage. Thank trachy for that one.
Source: Pocket Monsters

Comments

Actually, that damage is incorrect. The actual answer would be any Pokemon with an attack that says "Flip a coin until you get tails and this attack does x damage times the number of heads." If you never flip tails, then the damage will be infinite.