Both Warcraft and Starcraft. Don't get me wrong, I loved both of them, but Blizzard has basically admitted that they borrowed a bunch of their ideas from WarHammer and 40K.
On the subject of Halo, I don't think the story was as bad as most people think (at least not the first one), but it's really nothing special. Actually, the writers made one crucial mistake that could have given Halo an amazing back story. The book The Fall of Reach is to this day one of my favorite science fiction novels (granted, not as good as Ender's Game, but nothing ever will be), but the writers decided that the games and books would take place in parallel universes instead of the same one. As far as the games go, the books are non-canon. However, if this weren't the case and the events of the books were reflected in the games, Halo's storyline would have a lot more depth and originality to it.
Also, the first Halo was the only one of the games to have a decent story. The second was effectively a retelling of the first from the Covenant's prospective, and I have no idea what was up with the 3rd. In Halo 2, it would have been great if for the majority of the game, you worked with the defense of Earth, but nooooo. After just three levels of it, they chuck you back onto - guess what? - another Halo!. It spirals completely downhill from there, and the slope gets even steeper when they make you play as the Arbiter. Will he fight against human marines in an attempt to take over Earth? No, he's going to fight heretics who have never appeared in the story before and predictably never will again. And Halo 3's story was just a gigantic mess of cliches and predictability. Even though I enjoyed it more than Halo 2, that had nothing to do with the story. Oh dear god no...
Lastly, the Flood are headcrabs. End of story. As much as I enjoyed fighting them (again, at least in the first Halo), they are headcrabs. That is all.