Battle of the regional bugs!
Butterfree was the original butterfly pokemon that we grew up with and loved. I used one in LeafGreen version on my main team, and never regretted it. I also use it in competitive battling for the rare accurate sleep + U-turn combo.
Beedrill is the much tougher and more scary alternative. I used one on my Silver team (because I didn't understand at the time that he was weak, but I grew to love him anyway) and one of the most important characters in my fanfiction story has a Beedrill, so Beedrill and I have ties that run deep 
...but not as deep as Beautifly and Dustox. My first pokemon game was Sapphire version, and I honestly had no idea which pokemon were good or not. I basically only used 2 pokemon, Beautifly and Swampert for the vast majority of the game, and Wormy (Beautifly) had a mate named Casty (Dustox) that I started to train up at the end as well. Sadly, that game got erased, but Wormy and Casty are still very dear to my heart for being some of my very first pokemon ever.
Leavanny is a cute idea, but I don't have any particular attachment to him since he's new. Same goes for Scolipede minus the cute part. I do really like the design of Whirlipede, though. It's the only cocoon pokemon I would consider not evolving.
Vivilion is new, and seems like a hybrid of Venomoth and Mothim. Not that I mind, because he looks good. I suppose there are only so many variations on a butterfly you can do. I will still consider using him if I play XY version
