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.)
Happy - has 2 syllables, that's why "Happiest" is its Superlative. (You change "y" to "i" and add -est after the verb to make it in Superlative Degree.)
Easy-Easiest Nice-Nicest Truthful-Most Truthful
Generous-Most Generous Near-Nearest Helpful-Most Helpful
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.