For me Six Feet Under for a multitude of reasons.
It was given a few extra minutes to its running time to allow it to breathe properly, and while many credit its closing moments, I felt that it had the benefit of coming off of a solid final season. But for me, the true ending was the third to last episode, when Nate was laid to rest.
Someone created a post complaining about TV shoes and movie writers who try so hard to be clever that they end up undermining their work. What I liked about Six Feet Under was that the writers weren't trying to be clever. They left few unresolved plots, and the ones that did go unanswered were done so in service to the plot. You never found out who killed Nate's first wife, but you do have an inkling, even though it's never given satisfying closure, and it plays out in an episode of the final season, where he struggles to come to terms with that all the while struggling with the moral dilemma of explaining to his daughter what happened to her mother.
The final season should really be a master class on how to do a final season of a TV series -- or at the very least how to do a finale that will satisfy most fans. They didn't write themselves into a wall by introducing too many new character or plot elements into the mix, and most of the secondary plots from the previous season were resolved by the middle of the final season. So instead of pissing away the entire season on nonsense that went nowhere, they could devote the rest of the season to the main plot, which was Nate's impending death and the fallout. Most other shows that I have seen waste so much valuable time and then have to shoe horn everything into the final two episodes. It's very frustrating and it happens all too often.