Favorite Halo Game?

A friend of mine keeps trying to get me to play Halo, but I don't want to buy any new games now. He just won't stop, so most likely I will buy something Halo related for Christmas. What's your favorite Halo game or which Halo game would you recommend most?

If you're new to Halo, you might try Halo 5: Guardians.  Right now, this game is on pre-order if you have Xbox One.  There's a $10 coupon on the official Microsoft site on the game page that is valid during the month of December.