For me, it's a tie between Higurashi No Naku Koro Ni Kai (it's just so fucking satisfying and wraps up the entire series beautifully), the ORIGINAL FMA (I hate Brotherhood. Too shonen and I seriously believe that FMA shouldn't have a happy ending, but rather a bittersweet one), Cowboy Bebop for obvious reasons, and Neon Genesis Evangelion, once I understood it.
I still will never understand the overpowering love for Brotherhood and its "friendship conquers the day as we punch a god in the face" ending.