I'm a huge fan of Halo. I'm definitely interested in seeing how 343i Studios will continue the story, and introduce their new multi-player.
Personally, I categorize my favorite franchises between two types: Classic and Legendary. In my opinion, I classify Battlefield, Halo, Legend of Zelda titles as legendary franchises.
Halo provides a cinematic and deep storyline, that just gets better and better and better between each game, and overall, it allows for a interesting storyline that you never want to end. The multi-player is really fun in my opinion, and I have had great and classic matches and moments in every game, like Battlefield.
Overall, I title Halo as one of the best game franchises in gaming history, and I can't wait for Halo 4.