Disneyland or Magic Kingom, One day only.

[EpcotServo]

DISNEYLAND! Not even a fight.

 

[tirian]

For one day, I'd pick DL if you've never been there, because WDW requires days to fully enjoy. The MK spreads out attractions among WDW parks in favor of open walkways and landscaping (Tokyo DL and DL Paris do this, too). DL, on the other hand, is crammed with attractions because the resort only had one park for decades. This isn't bad; it's charming! If you've already experienced most of the MK attractions at WDW, focus on the truly unique ones at DL: the Matterhorn, PotC, the original Tiki Room, and Indiana Jones. The subs are decent, but the Seas with Nemo at Epcot is actually a more enjoyable attraction because you can see what's going on!

Off Topic: I still prefer the MK to DL, though, because I'd rather be able to walk freely than rejoice at a higher number of attractions in one park.