While not new to the series, as it was new in Smash 4, I like the beehive item a lot. It comes from the Animal Crossing games. If you shake a tree, every now and then, a beehive will fall. Attacking the villager and messing up his/her face for some time.
It's a throwing item and one of the best in the game. It's often a race between two players to grab it.
And it packs a punch when it lands.
Trust me, these bees won't leave you alone for awhile. Ouch, ouch, ouch!