At MK, if you have little kids, you should start in Fantasyland, to get it over with before the crowds hit. We don't have little kids, so we always start with Space Mountain!
At Epcot, we do Test Track first, then go back to Spaceship Earth (because the Spaceship Earth line goes really fast, even though it looks long). After that we wing it - usually go to the Living Seas next.
At MGM - we grab a Fast Pass at Tower of Terror, then go ride RnRR, then grab a Fast Pass for RnRR and ride Tower of Terror, then back to RnRR again! LOL! After that, we're worn out and need a break, so we get breakfast at the ABC Commissary! After breakfast, we do the Muppets while we're in the area, then do the Great Movie Ride.
AK - Safari first, then Countdown to Extinction, breakfast at Breakfastosaurus, then the Bug movie!