For me, it's no contest- The Voyage Home. Much of Final Frontier also qualifies, the general tone of Wrath of Khan is good, Kirk is excellent in Search For Spock, and many lines in Undiscovered Country are very *wink wink*, but it's The Voyage Home that really feels the most to me like a real extension of the serious & funny that defined the greatest show on Earth.
What's your pick?

What's your pick?