How would we form the superlative of the adjective common?
I thought it was most common, but my spelling checker recognizes commonest too.
I had the same question and so an upvote :)
This is how software tools are designed including MS Word. Don't worry, commonest is the word and many dictionaries define it.
commonest (adj) - Occurring, found, or done often; prevalent.
However, if you find it on Ngram, most common is more popular than commonest especially in recent years.
However, interesting point is, many people prefer speaking most common over commonest because if you say commonest problem it may sound communist problem! To avoid such ambiguity with such noun, prefer using most common.
There are two systems for the forms of comparisons of adjectives: One-syllable adjectives such as long have long, longer, longest. Adjectives with three and more syllables such as curious have curious, more curious, most curious. For two-syllable adjectives there is no simple and rigid rule. Grammarians have listed some endings where system 1 is to be used. But, I think, people don't have this list of endings in mind. Though grammars say it is common, commoner, commonest people prefer common, more common, most common.
I don't think that the word communist has an influence in this matter. With "communist" you use different structures. Either you say: He is a communist - or you say: the communist system.
I think people prefer "more common, most common" because it is easier to speak. Say three times "commoner, commonest" then you probably feel that hasn't the right "flow", somehow the two syllables with a weak vowel at the end are against the flow of speaking.