I suppose the decision for me is whether to take "series" to be the US meaning (the whole TV show) or the British meaning (just one season, in US parlance). Wellll I suppose what I could do is talk about my favourite series (US meaning) finale for a US show and favourite series (British meaning) finale for a British show, just to be controversial. ;-).
Favourite series finale of a US show
I might as well admit right now that I'm a Trekkie. Deep Space Nine is by far favourite of the 5 series because the fact that the show was set on a space station rather than a space ship there was much more scope for character development and also much longer story arcs.
The final episode ends a nine episode arc and opens with most of the major characters spending time with their significant others and families before a final battle with the Dominion. Before the credits, it's revealed that the Defiant isn't fighting on its own, but in a massive fleet of Federation, Klingon and Romulan ships:
The episode explores various themes, including The Cardissian resistance against the Dominion (as led by Damar and supported by Colonel Kira), the tension between the Cardissian forces and their supposed allies the Breen and Jem Hader, the development of both Sisko's and Kai Winn's spirituality and the relationship between Odo and his people, The Founders.
The first half of the episode is dedicated to the battle for Cardassia, in which the Cardassians wind up firing on their allies and joining the Federation fleet after the Dominion starts indiscriminately killing civilians in response to the resistance. Odo then convinces the female founder to surrender and all seems peaceful at last. However, unbeknownst to those celebrating the end of the war, Dukat and Kai Winn have been setting about freeing the Pah-Wraiths, the non-corporeal enemies of Bajoran Prophets. Sisko is alerted to this and confronts Dukat, and ultimately apparently gets lost in a fire. It transpires thought that the Prophets save him before he's lost in the flames and they then take him away from his pregnant wife and son as they still have much for him to do.
The episode ends with flashbacks from the last seven seasons, including a rather slash heavy one between O'Brien and Bashir.
After watching the finale for the benefits of this post I feel incredibly sad that the producers didn't see fit to continue the characters stories in canon.
Favourite series finale of a British show
|Doctor Who: Doomsday|
Well my favourite series of Doctor Who since it returned to BBC 1 in 2005 was the only series where David Tennant's tenure as the Doctor and Billie Piper's tenure as an assistant intersected. In retrospect, it seems amazing that they only had 13 episodes together because the will they won't they aspect of their characters' interplay had so many twists and turns that it's hard to believe they all occurred in such a short space of time (yes I know this space of time was extended by her various guest appearances since but shhhh).
In this episode both the cybermen and the daleks invade the earth. The cybermen wanted to make the whole human race into emotionless robots whereas the daleks just wanted to restore their race and destroy all other lifeforms. So, basically... the same as they always do!
While this was going on, Rose and the Doctor were busy being awesome in different parts of the Torchwood building. Then the Doctor gets kidnapped and taken to another universe and he is told that the story from their side. Basically, their universe is dying thanks to the jumping through a rift between the two universes in between which lies a void. This presents quite a quandary for the Doctor. To save both of the universes and pull the cybermen and daleks back to the void, he will have to shut the rift. This would enforce a separation between Rose and the Doctor because of timey-wimey things I can't be bothered to explain here. :-p.
Well Rose being Rose, she resists this after basically declaring how much she loves him and helps the Doctor close the rift, even risking her own life in the process. She is saved by her alternate father at the end and taken back to his universe. Then comes the real kicker. In order to say good bye, the Doctor beams his image to the universe she is now in.
There is a tearful good bye and Rose finally says the words "I love you". The Doctor is about to return the favour but takes his sweet time so doesn't get a chance to get the words out.