Definitions of "Nice" (from Websters)
1) agreeable; pleasant; delightful (POR)
2) attractive; pretty (POLY)
3) courteous and considerate (GF)
4) conforming to approved social standards; respectable (WL)
5) in good taste (BWV - My Home Resort - I could easily put any of the
DVC resorts here

)
6) good; excellent (AKL)
The joy of WDW resorts is that each is "nicest" in some way (although in some cases it's a more meaningful way to each individual). I, personally could not simply pick "nicest", as I like so many for such different reasons. In addition to the "definitions," here's a couple of thoughts:
GF is the most "refined" resort and offers the most service for the upscale traveler. That said, I would choose to stay at AKL or WL over GF, not because I do not like "upscale" travel, but because I believe AKL and WL are different than anything you'll find elsewhere...their unique and majestic nature make them "nicest."
Finally, there is nothing "nicer" than the joy of coming back to a 2 BR apartment at one of the DVC resorts...but that's opens up a whole different can of worms
Given these thoughts, it would follow that VWL should win, but since I have yet to stay there, I cannot possibly make such an ultimate determination (and, besides, my fellow BWV owners might string me up!

)