Definately Red v.s. Blue, mainly because aside from the graphic novels (which are available for essentially any best-selling game) I hadn't heard of the other "huge" events.
What I do think helped the Halo franchise become popular was it's use of Xbox Live. Simply the storyline and offline multiplayer wouldn't have been enough in my eyes to carry the series, but when you then have a console game with up to 32 players fighting at the same time, then that's the sort of thing anyone would want to be part of.
Well, except PC gamers. They've been pwning since Master Chief was doing space toilet training.