If you're asking the highest BST of a Pokémon who hasn't evolved into it's final form, but has evolved from a prior stage, then that would be Porygon2, which has a BST of 515.
However if you're asking the highest BST of a Pokémon who can evolve, but hasn't evolved at all, then that would be Scyther, which has a BST of 500.
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