Superlative form of chatty: most chatty.(adjective)
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Use "chattiest" in a sentence
"Comparing all of our friends and followers shows us that the chattiest group (highest average number of tweets per user) are those which are our friends-but-not-followers."
"This was the chattiest guy I've run into in a long time."
"Cuz you know, I can talk the ears off the chattiest toddler and teach her all the really cool things in life at the same time."