So difficult to choose because Nintendo did such an excellent job putting them together but I think mine would go to Palutena's.
Kid Icarus: Uprising is my favorite game Nintendo has released since Brawl or Pikmin 2 and hearing her and Pit hold conversation together brought back some serious nostalgic memories of my first play through of the game. Especially when she introduces herself as a character, "I'm here for a little.. uh, divine intervention!". Then the section where she lists all of her moves and they show other charcters in Smash 4, "Many worthy foes have gathered here.. and I won't hold back on anyone.." The music from Kid Icarus sounds so perfect when brought forth at a particular section there.
It wasn't as "out of nowhere" or left field as say Megaman's, Greninja's or Robin's but Palutena's character was captured so well and I wanted her in from the start. Her demeanor is exactly how I envisioned her in Smash. I remember my friends waking me out of bed and screaming that she was confirmed on the morning of E3, at first I thought they were trolling lol. Plus it created the
WOOP WOOP! Palutena pullover vid, and we all know how much that's worth.
If I had to rank them, I'd do this order (can't leave any of them out, love them all so much):
1. Palutena
2. Little Mac
3. Robin + Lucina
4. Shulk
5. Megaman + Villager
6. Greninja + Charizard
7. Pac Man
8. Wii Fit Trainer
9. Rosalina
10. Miis