Haha, when entering the post I thought that you were referring to the Ending animated sequence (+music) of animes.
But now on the topic: The best ending for me is an actual ending (series/story finale), so I can't concur with Re:Zero as it just leaves everything open (even if the execution was good, it just a "season finale"). And I think the same with Geass as Lelouch is the Schrödinger's cat of anime with that ending.
My picks:
Clannad: After all the crying you feel good and relieved.
Boku dake ga Inai Machi: The whole series is well structured and executed, leaving nothing out.
Working!!!: All good things come to an end.
Fullmetal Alchemist: You feel that the epic story of war, death, conspiracy and sorrow around the alchemist Elric brothers truly ends, and what follows (that we will not see) is just the story of the Elric family, just another family story.