As of 2014, Ash has never kissed Misty in the Pokémon anime TV show. According to Pokeshipping, it is a popular false myth that there is an episode, movie or some sort of secret footage showing the pair kissing.
In one of the movie shorts, Ash and Misty face each other, and they come close together. One could interpret that as an action before or after a kiss, but no kiss was shown on camera. There is a CD by an American company called "Pokémon Christmas Bash." A song called "Under the Mistletoe" exists on that CD which plays sounds that could be interpreted as a kiss. However, it is not in the TV show.