There is no clear answer because it depend on how much you attach to the characters and series, otherwise you won't feel a thing for saddest moment in fiction if you don't care about them.
In my opinion If it was about one companion than Brock at the end of DP, he was with Ash for longest time and witness all of his growth till DP (except Orange island), it really hit when Brock said Pokemon master Ash and Ash say Pokemon doctor Brock.
If it was about the ending than SM, Ash first time not just say goodbye to his friends but also his family, his pokemons and his home too, after OS this was first time Ash cried, the family hug scene hit really hard, even TR first time got to say goodbye to their Pokemons at the end, not to mention Ash leave the region as a champion to take on world challenge rather than someone who lose another league, also how Ash was struggling to reveal about his departure to everyone and his speech to the group was very emotional, Lillie departure scene, how Mallow saying goodbye to all of her friends with smile perfectly show her character who always try her best to make other happy while hiding her own sadness.
Again it's all personal preference as to what series you attached the most but in my opinion SM handle it the best, all other goodbye had at least one major flaw while SM goodbye had minor.