HARBINGER (MASS EFFECT 3)
The Reapers are the biggest of the big bads, with their purpose being to wipe out literally all sentient life in the galaxy. As the leader of these terrifying beings, there should be no one more terrifying than Harbinger- who, sure enough, is built up as such over the course of two games. Except, you never get to fight him. Hell, during the conclusion of Mass Effect 3, you never even see him. You hear about him and his forces approaching Earth over the radio, but your “final battle”, so to speak, is a binary dialogue choice in the Citadel. There are many reasons Mass Effect 3’s ending is considered disappointing, and this is surely one of them.