"A diversion!" Ack. Wrecks the scene.
I know there are more but I haven't seen any of the movies in several months now.
The cheesiest scene would have to be when Frodo wakes up in the Fields of Cormallen...
*Frodo wakes up, sees Gandalf and says in a dumb voice "Gan - dalf?"
Enter Merry and Pippin (I think), who *leap* on the bed.
Slowly the fellowship appears in the room, as the scene gets progressively worse. Even the music is cheesy. What could be a very sweet and moving scene became an extremely cheesy (and in turn, extremely hilarious) scene.*
The only scene that is nearly as bad is the scene when the Hobbits reunite in Rivendell - cheesy laughter, exaggerated movements...
(Those aren't really lines but they fit the bill).