As with most things it depends on the child. They all have pros on cons based on the child.
We took our son when he was 18 months and then again a month before turning 3.
The first time he was skiddish with a lot of rides (liked Dumbo, Figment, Safari; hated TS Mania, Carousel), but he loved the characters (so character meals are great for that). He also spent 20 solid minutes looking at the bricks outside MK (until we pulled him away).
The second time he wanted to do more rides (and some rides againa nd again) while was slightly less friendly with some of the characters.
He was also VERY familiar with the Disney movies and recognized many of the characters by trip #2. He was into animals so AK was a total win. We would usually hit up Epcot at nap time so we can stroll (and enjoy some F&W snacks while he snoozed).
HS has Disney Junior and Muppets if they are familiar with them - otherwise I might pass on it.
MK obviously ahs the most rides and characters and things to do, but schedule in a FP or two or risk waiting in a line with a 2 yr old. Carousel of progress, peoplemover, small world are always good quieter, cooler ridea to catch that entertaining break
AK has lots to see visually, from playing with musical intsruments in a store to seeing animals all over. There are a good amount of kid-friendly rides as well as a fe shows.
EP is best for nap time. Get them to crash in the stroller than grab a margarita and guac and enjoy the adult time. There are a couple of slow, quiet, cool rides to catch a break and keep them entertained at the same time.
They probably won't remember this trip, but you'll never forget it.