I think there's something great about each of these episodes, but my favorite would have to be Pixie Village because we see the epic showdown between Icy and Princess Amentia, we get some shipping moments between Palladium and Avalon, and we are introduced to Charmix with Bloom being the first of the Winx to earn hers.
I almost chose Charmix Power because we get a few cute scenes between Musa and Riven but there's just a lot more going for Pixie Village.