Answer:
The card that Cora bought for me was the most thoughtful  one of all.
Explanation:
The words 'of all' suggests three or more things.
The rule says to use the superlative form for three or more things.
The superlative form is either:
>  the + adjective + est   (for 1- or 2-syllable adjectives -- though there are exceptions)
>  the + most + adjective  (for multisyllabic adjectives  -- though there are exceptions)