Always do the rides with the worst lines first, assuming you are going to be in the park at opening, which I strongly suggest.
So in terms of the Magic Kingdom, that would be either Splash Mountain or Space Mountain. If you do Splash first, head immediately to Big Thunder Mountain, then head across to Space. If you start with Space, then head to Frontierland to get the other two mountains out of the way.
In Epcot, start with Soaring, then head to Test Track.
At DHS, start with Toy Story Mania, then Rockin Roller Coaster, then Tower of Terror.
At AK, start with Expedition Everest, then head toward Kilamanjaro Safari.
All of this assumes you do NOT use Fastpass. An alternative in every park is to send your fastest member to pickup a Fastpass for one attraction, then head to the other headliner. As an alternative, pick up a Fastpass for whichever line you are in first, so you can ride a second time quickly without waiting.
Have fun!