My vote goes to Lost. Basically, Harchi suffers from separation anxiety issues and Popy and Buck think Harchi is dead at one point. I've never seen Poppy so sad. She actually CRIED in that episode.

A Manolo's Best Friend is pretty jarring, considering a tough dog like Roco is reduced to a sobbing mess but I'm still casting my vote for Lost, being a big Harchi fan and all.