Overrated: Halo and Halo 2. Fun, but people talk about these games like they revolutionized video games forever when there have been FPS games that had been doing the same thing for years.
Halo did do some stuff different. The biggest thing it added was enemy AI. They were more than mindless drones. They used tactics to try to kill you. Halo was a game changer.
They also opened up the world of online multiplayer gaming for home consoles. Dreamcast did it before, successfully. But the Dreamcast didn't have nearly as large of a fanbase, and it was dead in the water by then.
But more than your point or my own. The reason I love those games so much is the story. It plays-out like a sci-fi blockbuster movie that just keeps you on the edge of your seat, especially Halo 2. Slick graphics, great voice-overs, and stellar soundtrack add to that feeling, too. And I loved in Halo 2 how you saw the story from both Master Chief's and The Arbitur's perspectives.