I'd say The Voyage Home, where he's Shatner's bubbly self instead of a starship captain.
Even though TVH was a comedy, I didn't get the sense that Shatner was camping it up and winking and nudging the way he did in, let's say Airplane II. You're supposed to laugh with him, not at him.
The problem I have with V is that Kirk treats the bridge like it's his personal living room, and the other veterans are his friends, and the rest of the crew are peons. He strolls on with his bomber jacket and just kind of sits down, oh, hum, another day at the office, after having climbed the mountain. I just don't sense that buttoned-down Master and Commander aspect that was there in Trek II, which I think is a core part of who Kirk should be.