Halo 4 - deepest and best single player in the series, it had me and my brother gripped all though the game.
Halo 3 - an amazing end of the first trilogy, great multi-player and an even better single player. Only downside is that the story isn't as involving as I would have liked, still great though.
Halo 5 - by far the best multi-player but weaker on the single player side. Still the Halo game I have sunk the most hours into, by a large margin