Currently it would have to be the Yuzihara Town contest. The interaction between all the characters was quite enjoyable, and we got to see a side of Jessie that (these days at least) is rarely shown.
That and the fact that Combusken put up one hell of fight against Wakana's Vibrava, which made the episode for me.
Though disregarding my bias for Combusken, the Modama Town contest vs. Harley would be a definate winner. Superb animation, many hillarious scenes, an excellent battle involving an interesting, yet shafted creature taking down one of the characters that I dislike most and of course, the ever welcome addition of one of the best antagonists we've had in years.