I really like the unique dialogue in Wedding Day Massacre, and Son I'll probably fight a lot since he's a new raid. In terms of story mission I have the most fun playing Wattle Gobbler, 'cause Hunger Games.
But largely thanks to the loot train, I went with Mercenary Day. I also like the charm of the area, down to the lootable gift boxes. The little model of Gingerton with its toot-tooting train had me gushing and set a good tone.