My favourite so far has to be the fourth episode, The Battle Club and Tepig's Choice. I'm sorry, but I'm a sucker for Ash being all caring and nice towards Pokemon, especially ones that have been neglected or abandoned. D'aaw.
I also liked episode 19 (the one with Cabernet), and episode 20. I thought the Ducklett were hilarious for some reason. And, I also liked episode 29, because I loved the Team Rocket/Ash&Co. teamwork. I have a feeling once the Team Plasma vs. Team Rocket episodes are aired, they might find a spot in my favourite BW episodes too. :)