I remember MS word suggesting the words most common to commonest and it still does! That's okay. Why use two words when a single sure-word is available for that (Save ink, save energy from typing)
But then, are there any cases where splitting out the adjectives is preferred. Interesting is Ngram for the usage most common over commonest in recent years. The result is surprising.
Ngram results apart, is there something that decides most common over commonest? Also, my question is not limited to most common and commonest but all such superlatives.