I watched this episode for the first time in nearly four years and I'll say this: Unusual choice for music when Butterfree departs.
Anyway, seeing Team Rocket when tehy get scared that Pikachu is going to shock them is still hilarious and never fails to amuse me. But, the defining moment would be Butterfree leaving, it made a 21-year-old cry even after all these years, but Primeape, Charizard, Piegeot leaving didn't make me cry in the past, go figure.
For me, this applies "semi". I did cry when Ash's Charizard, Squirtle, Lapras and Pidgeot left (but that was nothing compared to Butterfree), but when, for example, Ash's Gliscor left, I didn't care.