As the series went forward the actors seemed to stop playing their characters and start playing themselves. This was never more evident than with Shatner himself, who undergoes a definite shift from "Kirk" to "Shatner" somewhere around The Voyage Home. Possibly even as early as TSFS.
I'd say The Voyage Home, where he's Shatner's bubbly self instead of a starship captain. Undiscovered Country sees him return to playing Kirk, even if the joke about Kirk always having wanted to kiss himself was a joke regarding Shatner's media persona, as it clearly didn't fit the Kirk character at all.