Normally, my vote would go with the Polynesian. Just love the theming. However, with a lot of the better buildings converted to
DVC, make sure you get a room with a full balcony or patio. A lot of the older buildings have whole floors with neither. It is a really convenient resort though with Monorail to many food venues at GF, CR and MK--you can also take boat from CR to WL. If you don't mind walking, you can walk over to the TTC and get the Monorail to EPCOT directly instead of boarding at the hotel and having to switch.
All that said, I also really like the AKL. The lobby alone is a WOW factor for anyone! I had a great savannah view room with the highlight being the many giraffes outside my balcony. Many other animal sightings from the viewing areas around the resorts as well--ostriches, zebras, impalas, etc. It's a lot quieter than the Poly. The bus service was great when I was there. You have to take buses everywhere--no walking or Monorail to parks. Beautiful pool area too.
I do not like the BC. IMHO it was one of the least themed. It just didn't feel Disney-special to me. I know lots, lots, lots, and lots of people will disagree, but I stand by votes that Poly and AKL are much better. Of course, I can't deny the benefits--the pool area (if you intend to spend a lot of time there--is spectacular compared to other resorts--though AKL's pool is pretty sweet too) and the proximity to EPCOT (I was very luck in that regard--my room was right near the end of the building closest to the International entrance.). If I would stay again in the EPCOT resorts, I think I would choose the Boardwalk. I really enjoyed it because it had a more intimate feel and I liked the decor a bit more than BC. I may try the YC, just because I haven't stayed there before, but I think I would be just as disappointed as I was with BC when it comes to room decor. And it is a very, very long building.
REMEMBER--These are just my humble opinions!!!!!!!
You'll have a great time no matter where you decide--have loads of fun!