I will introduce a dark horse into the competition in the form of Forever Knight. FK was a show about a 500+ year old French vampire in Canada who becomes a police officer in an effort to make up for all the bad he's done over the centuries. The third season was pretty weak and it was to be their last, but they made the most of it and ended on a bang.
Our hero Nick Night (Forever Knight, get it?) had figured out it was possible for a vampire to regain his mortality by draining a mortal in a very specific way that would leave both of them alive. The problem is that this is very, very dangerous and extremely unlikely to work. This is a callback to season two where we met a former vampire friend of Nick's who is now mortal because of this process. Nick's gal pal and love interest Natalie volunteers and things go as well as you expect. Nick accidentally kills Natalie in the process and he's still a vampire.
So Nick turns to his sire, and occasional antagonist throughout the series, LaCroix, asking him to stake him through the heart and put himself out of his misery.
LaCroix (prepares to stake Nick): I suppose I'm the bad guy in your tale?
Nick: You were my best friend.
LaCroix: Damn you, Nicholas. (Stabs Nick)
I think it took a lot of guts to end the series with such finality.