Feast of Winter Veil is nice and Brewfest is fun (though a disappointment over last year's), but my favorite holiday this year in Azeroth is Hallow's End. The Midsummer Fire Festival is a close second, mind you. It was fun and profitable. But Hallow's End has better lore and better accessories.
Sure, I love my dancing flames chickie, but my Rickety Magic Broom is the best thing ever -- and it's only temporary. The masks, the candy, the pumpkin heads -- it's just fun.