I'll echo a few that were already posted. I'd been 20 times and had never seen the Finding Nemo stage show until my last trip with a fellow adult instead of my teenage son, who always wanted to bypass it. I LOVED it! Definitely see that one. I agree that the entertainers in Africa at Animal Kingdom are also wonderful.
In Epcot, World Showcase performers are great too. I particularly love the Dragon Chinese Acrobats and the Japanese drummers.
We are headed back in June too and this post makes me want to see the Frozen Sing-Along but there's no way my 14-year-old son will go for that. I can't even get him to agree to the Frozen ride.