I think the largest part is going to depend on
what your child can ride and what they might
like riding. We just got back from a week long trip with our almost 3 year old daughter. We did 3 days MK, 2 Epcot, 1 AK, and 1 HS. We took a midday break each day, got to the parks early but didn't kill ourselves trying to rope drop, and stayed out as late as we felt/DD could handle, and she was a trooper. We really only had one or two meltdowns the entire trip and they were on warmer days toward the end when I think the cumulative effect was just starting to get to her. DD is also on the petite side, probably right at 3 feet tall, which limited the rides she could go on, especially at HS.
MK was obviously her favorite and that's to be expected. She liked pretty much everything we took her on. We wanted to do a few more things but even with 3 days, we couldn't quite hit it all, though there is only one thing off the top of my head that we had wanted to do and didn't get to.
At Epcot she loved Frozen and Remy, and enjoyed the others, though they were a bit slow paced for her. Had she been tall enough I really think that she would have enjoyed Soarin and maybe even Test Track. After we got home she kept talking about rides where she got to go fast.
Even though the number of rides was very small, only three, at HS she still really enjoyed the park. She loves Star Wars like dad and so she really enjoyed going through Galaxy's Edge, seeing the ships, and she got to have a fantastic interaction with Rey and Chewbacca that was one of the highlights of the trip.
AK was also a lot of fun for her even though we really only took her on the safari and Navi river and did a few of the shows. Had she been about 2 inches taller that would have opened up a few more rides, and 4 inches would have allowed several more. She is adventurous and after we showed her videos of Barnstormer on
youtube she was all in for that and rode it 4 or 5 times and is still talking about it now that we're home. I have no doubt if she had been tall enough she would have been game to try Thunder, Space, and other roller coasters, too, which is why I mentioned above about what your child can ride and what they might want to ride.