"Who Done It", the second part of the Who Shot JR episodes from Dallas, is still the second most-watched television episode of all time. Behind only the finale of MASH. 76% of all televisions were tuned to that episode. It's literally credited for popularizing the cliffhanger, and helping CNN get off of the ground.

Hard to beat that.

https://en.wikipedia.org/wiki/Who_shot_J.R.%3F