Casually, it would have to be Mew, because that was totally not obvious. But seriously, Mew has long been my favorite Pokémon. I love how I does what it wants and doesn't care what anyone else thinks or does. It can be quite unpredictable in battle with its ability to learn every single TM out there. Also, it's a kitty! Competitively, it would have to be Florges. My Florges on my multiple competitive builds has been known to wreck everything when I really needed it, especially when my team is in bad shape. I love how bulky it is, and how it can stall out pretty much anything, if used correctly.