If there a TV series where the last series is definitely the best?

[Hasbro]

I'm not sure of teh total run of Newhart, but its first season was widely considered the weakest, then they figured it out and did some recasting. The finale is in the pantheon and the last season has my favorite episode (when the reform their old street gang the Holligans).

 

[trojansoilers]

Supernatural Season 5

It still amazes me how they managed to fill in 10 more seasons of storyline after that near-perfect Season 5 finale.

 

[Praetorian Caps]

Game of Thrones!

Ducks and runs for cover.
I still haven’t watched the final season.

I watched it for the first time last year and it wasn't as bad as it was made out to be. Could it have been better? Yes, but it tied up all the storylines in a satisfactory manner.

And to answer the OP, Angel's last season was its best and Not Fade Away is the best series finale of all time.