I also like Love is in the Air and Noblegarden, they're wonderfully corny and cute, with the bunny ears and the lovebirds. Winter Veil isn't bad either, but perhaps that's because I have a child-like love for Christmas.
But my favorite in-game holiday by a long shot is Brewfest. I really like the music, for a while at least. With my habit of alt-tabbing to work while leaving the game running, leaving a character standing in a Brewfest zone for long periods of time is annoying at best! The games are really fun, and the in-game drunkenness does bring a smile. If Coren Direbrew was as entertaining as the Horseman, and the rewards were as fun as the Lovebird or the Picnic Basket, this could be a perfect holiday!
What about you? Which in-game holiday tickles your fancy, and are there any that you despise?