My daughter's favorite last year (at 20 months) was Small World. She also enjoyed the Pooh ride, Dumbo, the Carousel, the Safari at AK, Playhouse Disney at DS.
Anything in fantasyland....except Snow White...ick.
It really is hard to go wrong at MK, if they are tall enough they should enjoy it. My kids all enjoyed the Haunted Mansion at a young age, once they get past the stretching room its not as scary.