Easy: Magic Kingdom.
EPCOT - WS is nice, but let's face it - Future World is a big concrete wasteland with a few interesting buildings around. OK, that's not really fair, but the sidewalk-to-interesting-object ratio is way out of whack.
MGM - the lack of a cohesive theme hurts it here, though there are some neat things and has some especially good night photography possibilities.
AK - great for animal photography, but last time I checked, AK is supposed to be "Nahtazu"! I can get animal photos at any ol' zoo (though usually not with such nice surroundings.) Still, AK is very close to the top pick - but the park usually closes before dark so not as much effect went into night lighting IMHO, so it's not as good for night shots.
DL/DCA - lots of neat opportunities but just too darned crowded most of the time, thanks to its "intimate" size. Also, being more of a "locals" park, it's easy to feel more conspicuous carrying around a pile of camera kit.
Tokyo DL - haven't been there but from what I've seen, this will top the list once I do go there.
So, that leaves WDW's Magic Kingdom - the park I grew up going to, with many great photography options, great night shot possibilities, and it's the only park that you can easily photograph from
outside the park - from the fire escape at the Contemporary, or the beach at the Poly or GF, or from the boat.
Oh yes, almost forgot, one more strike against MGM and AK - no monorails! It's just not proper Disney park photography without monorails.
