Its hard to say. Halo obviously has the richest lore, but Destiny has great potential. Also, the Borderlands universe is still in its infancy and is shaping up to be really cool and original.
Borderlands 2, Tiny Tina's Assault on Dragon Keep. I know, it's a DLC and not a full-on game, but it was very twisted in a good way. And the final boss, with the characters' confrontations... Wow.