The Superlative of the Verb is base on the syllables and compatibility of the pronunciation of the verb.
Intelligent - has 3 syllables, that's why "Most Intelligent" is its Superlative. (You put "most" before the verb to make it in Superlative Degree.)
Smart - has 1 syllable, that's why "Smartest" is its Superlative. (You connect -est after the verb to make it in Superlative Degree.)
Try to pronounce the possible Superlative word for the given word. If it's good to hear, it's correct. If not, it's wrong.