A Pokemon must have a base HP stat of 48 to have 300 HP with 252 HP EVs. A Pokemon with a base HP stat of 80 has over 300 HP without any investment.

A Pokemon must have a base HP stat of 98 to have 400 HP with 252 HP EVs. No Pokemon has a base HP of 98, so of current Pokemon you must have a base HP stat of 99 to have over 400 HP. A Pokemon with a base HP stat of 130 has over 400 HP without any investment.

A Pokemon must have a base HP stat of 148 to have 500 HP with 252 HP EVs. No Pokemon has a base HP of 148, so of current Pokemon you must have a base HP stat of 150 to have over 500 HP. Wobbuffet, Chansey, and Blissey are the only Pokemon that can have over 500 HP with no HP investment.

Blissey, Chansey, and Wobbuffet have the highest HP. Please refer to this link http://pokemondb.net/pokedex if you wish to see other Pokemon with high HP. Just click on the HP column.