I chose The Undiscovered Country because to me it felt like a true "Series Finale" of the TOS world. It dealt with many plot points that have been evident over TOS one. Kirk's animosity towards the Klingons killing his son, The Federation and the Klingons heading towards a truce of sorts. and the Enterprise finally being decommissioned (as mentioned in TSFS) and the crew (as well as the actors) moving on from what was basically the last 25 years of their life.