I wouldn't want to use the X-mas Wooper, just because it would make me think of Christmas every time I logged on. XD
Anyways, I like the Plusle's Play Date one. It's what I usually use. I haven't found any glitches in it yet, unlike most of the others. ^_^; I like Pokemon World Remix, but it looks messed-up on my computer. I also like Ralt's Dance, but sometime it bothers me because the colors are a bit bright, and the links look wierd sometimes, lol.
I can't wait for the Halloween Eevee one, though! =P I'm going to use it on my birthday. It looks so adorable.