5. The Master (Fallout)
Should you wish it to be the case, your showdown with The Master can play out like a particularly gruelling, traditional final boss fight that only the greatest of Fallout players will survive. However, it is the fact that The Master doesn't force players into that option that makes it so memorable.
The Master is responsible for pulling a great many strings throughout the course of Fallout, so it is appropriate that he would try to do the same when you finally meet him in the game's finale. Fight him if you wish, but also consider that it is possible to bypass him entirely, convince him to commit suicide or even join him by becoming a member of his Super Mutant army.
This element of choice not only fits The Master's role in the Fallout universe perfectly, it helps seal his legacy as perhaps the most compelling antagonist in gaming history by ensuring his final moments are just as unconventional as his haunting appearance and ominous motivations.