Pertaining to Magic Kingdom - my Mr. Toad die cast ride vehicle with Mr. Toad and Moley on board. Ride, Toad, ride! (Who cares that it is technically a DisneyLAND collection piece?!)
Okay, then - if you want to split hairs, my favorite all time souvenier from WDW's Magic Kingdom is a spooky rubber and felt "Grim Reaper" mask purchased from the now defunct House of Magic on Main St. USA. I had a lot of fun with that mask for many years until it fell to pieces from use.
From EPCOT: My Rose and Crown Bass Ale drink mug "pinched" by my girlfriend at the time and bestowed upon me as a gift. Oooh, so bad! :goodnevil As that was about 18 years ago, I feel confident the statute of limitations from prosecution has probably expired.
But maybe my all time favorite overall souvenier is - the glow in the dark frisbee from Sci-Fi Theater at the Disney-MGM Studios. It sits on a memorabilia shelf at the house in the guest bedroom and glows when the lights go off.