Okay, so my first choice: Butterfree! definitely not sucking up to Brendino
Simply because it's the original butterfly and it's really cute, and it evolves from Venonat and it has a great ability in Compound Eyes, boosting his status inflicting moves from 75% acc. to near 100%, AND boosting the odds of finding wild Pokemon with items. As a matter of fact, my Marshmallo (damn you, gen 5 character limit) the female Butterfree, was my main player today as I hunted Solrock for Sun Stones in Glittering Cave. Did I mention that it's also really cute?
and just to show that I'm definitely not sucking up to Brendino I also chose Volcarona, because it's looks beautiful, though I think that I'd be terrified if I saw one IRL. But the fan art is really gorgeous, so I really can't pass on voting for it as well. Plus, it's great in battle, as others have mentioned, and it also has Flame Body + Fly, a killer combo as any breeder would know.
And yeah, I also voted for Vivi, because they did a pretty interesting thing with all the wing patterns. I like that effect with the scales scattering from its wings when you view it in your party, with the scales actually matching the color of the wings, I thought it was a nice touch. Also, I find it really funny that they made Pokemon #666 such a cute and fun critter.