Definitely Buzz Lightyear. We rode it at least 13 times with our DS (5). Also, Mickey and Minnies Houses and Goofy's Barnstormer in Toontown.
Our son also loved dinner at Chef Mickey's, the Star Wars ride at MGM, and everything in the AK. I can't tell you how surprised I was, though, when he just wanted to hang out at the resort, play in the sand and swim. Sometimes they need downtime without all the visual stimulation you get in the parks