That's highly subjective. We really love the street entertainers and could spend the better part of a day running around a park to see their different shows, but I think a lot of people will respond with shows that have their own venue and are listed as attractions (like the rides) on park maps. Here's my picks:
MK: street entertainment-- Dapper Dans, flag retreat, ragtime pianist outside Casey's; attractions-- Hall of Presidents, Country Bear Jamboree (no live entertainment, all animatronics)
Epcot: street entertainment-- Voices of Liberty, Japan drum show, holiday storytellers at Christmas
DHS: Beauty and the Beast and The Little Mermaid attraction shows are okay in my book, worth seeing
AK: Here I like attraction shows best-- Finding Nemo (best attraction show on property), FotLK, Flights of Wonder; street entertainment always makes me happy here, but they change the shows a lot, so I'd try what sounds good, but I have no up-to-date recommendations
ALL fireworks shows top my list and are musts in my book-- Wishes, Illuminations, Fantasmic, and I can't wait to see the new Star Wars show.