I could pick the dance sequence, which taps into classic Ed Grimley moves, the moment in the freezer truck when Tuck manipulates Jack into action, or the ridiculousness of The Cowboy. All are great, but if I have to choose one, it would be the final moment in a collection of scenes where Tuck realizes that he's in a human man, Jack thinks he's going nuts, and how it all leads to the most logical conclusion in the doctor's office waiting room.
The clip after the jump starts just after the craziness with the cash register, and the real gem lies between 5:30 and 7:11. Jack parks his car and makes his way to the doctor's office just as Tuck makes aural contact. Jack desperately tries to convince himself that the people around him are the ones talking to him, until he finally stands up and screeches: "Oh God, somebody help me! I'M POSSESSED!"
Simple brilliance, I tells ya. What's your favorite Innerspace scene?