I say Sewaddle, cause Leavanny was a big force in my team (one of my various teams). I had it hold a Scope Lens, and with Shadow Claw, X-Scissor & Leaf Blade in it's moveset, it became a downright murderer.
For me, Sewaddle. It was a close call between it and Venipede. I like the latter's design and evolutions a lot, but ultimately Sewaddle's cuteness and typing makes me vote for it. Leavanny is also one of the coolest Pokémon in GenV in my opinion, which makes Sewaddle all the more attractive.
I chose Caterpie because I never used a bug pokemon and I did have a butterfree until like it was level 12 in HG but I may try out more just because Joltik is awesome so I'm starting to like bug types now. Partially why I hate them is because they're just so annoying especially in the first forest xD