This is a particularly tough question. I love WDW during the holidays. OK, so let's start off with a piece of advice that will make it easier to narrow the choices: Visit as many of the resorts as you can, and see their decorations as well. You're off to a good start, because for my money, the Wilderness Lodge just screams "Christmas." The tree in the lobby is... impressive is such a small word for such a big tree! You'll love wandering the resort and noticing the little Christmas touches. That said, be sure to head over to the Grand Floridian as well, because when that place puts on its Christmas finery, it's a sight to behold! I don't really think of the Poly as being all that incredibly Christmas-y. I do like some of the decorations over at The Boardwalk.
OK, now on to the resorts.
I suppose if I could only do one (ugh, this is tough!) I'd maybe suggest Epcot. NOt only do you get to see Christmas decorations from around the globe, but there's one of my favorite sights, as well: As you walk toward World Showcase, you'll notice that the lights of the International Archway are synchronized to the music playing for the fountain. I could sit and watch that for quite some time! Then of course there's the santas from around the world, and the tree-lighting ceremony. And, of course, there's the nightly Candlelight Processional.
You almost luck out, in that any other year, I'd put MGM at the top of my list if only because you will never, ever see anything quite as spectacular (and tacky at the same time... will wonders never cease?) as the Osborne Light Festival. But that's not being displayed this year, so MGM falls into a lower position by default.
Animal Kingdom is beautiful, but doesn't do Christmas to quite the extent that Epcot does. And of course seeing Main Street USA in the Magic Kingdom decorated for Christmas is beautiful, but... yes, I think when it comes to getting the most "holiday bang for your buck," I'd go with Epcot.
Anyone else?
Richard