I like it when the 'lee goes from giggles to a full on musical laugh without going into prolonged screams. Along with that is when the 'lee does some banter and attitude with the 'ler, who will then proceed to tickle the 'lee that will cause the laughter to go up a few decibels.
The classic example is Farra from Jeff's old MTP clips. A more recent set of examples are Xavier's models for Frenchtickling like Paige, Hortense, or the pair of Kelly and Cyrielle.