1. Pumpkaboo - IT'S A PUMPKIN WITH A BAT ON TOP OF IT! AND IT'S A GHOST-TYPE! AND IT COMES IN DIFFERENT SIZES AND CAN LEARN FLAME CHARGE FOR SOME REASON AND CAN TAKE THINGS TRICK-OR-TREATING TO MAKE THEM ALSO GHOSTS EEEE And I feel like I should also count Gourgeist here, because I like it for pretty much the same reasons except it's a candle instead of a bat and the colors are reversed. It sucks that I have to trade for it, though.
2. Floette - THIS THING IS ADORABLE! Seriously, I went the whole game without evolving mine because I liked it so much better that Florges. It made for some fun times at the end when I used her to floor AZ's last Pokemon.
3. Trevenant - Granted, I only actually used a Phantump in the game itself, but still, Forest's Curse led to some fun times. There's nothing quite like that feeling when you realize you almost oneshotted your version legendary because you forgot you made it double-weak to Poison! That aside, it's also a haunted tree, and that's just awesome.
4. Carbink - I tried to avoid learning about new Pokemon in advance as much as possible, so I was surprised by this li'l guy. The fact that it couldn't evolve was a turnoff, but still, I ended up considering it for my party all the way up to the point where I finalized things. If I ever get another playthrough, I'm not going to pass it up again!
5. Meowstic (F) - This was a tough one, actually. I went with Meowstic because I like her design the best, but I actually dropped mine from my ingame team because it seemed like she had so much overlap: my Delphox (who I wasn't dropping) could also cover Psychic attacks, the Charge Beam she learned around when she evolved was obsoleted because of my Helioptile, and the Shadow Ball she learned a few levels later didn't tilt things in her favor, since I already knew I wanted to keep an eye out for Ghosts to use. Also, I'm pretty sure she also had an Impish nature. Like Carbink, I didn't stick with her, but I think I'd very much like to on another playthrough.
Other contenders for the #5 spot included Braixen, Slurpuff, and Klefki. --- Official Bride and Wife of Noire (of the Fire Emblem Awakening message board)