HS - Toy Story Mania - and you need to SPRINT for it!!! Even the line for the fast pass machines is unbelievable at park opening time!!!! Luckily as everyone stays there for a while, once you get your fast pass you can go straight on the other fast pass rides - Tower of Terror and RnR Coaster, with barely a wait!
MK - I'm probably not much help here as I tend to stay mainly round Fantasy Land so use my fastpasses in there! I imagine most people will say Splash Mountain!
Epcot - everyone goes straight for Soarin, so I would do that to get a FP, then rush over to Test Track (my fave ride in any park!) and Mission Space which will still have minimal lines
AK - We always rush to the safari to get a nice early fastpass time, as most of the animals are out early. If you like thrill rides though you will probably head for EE instead!
To be honest - with the exception of TSM - if you are at the park for opening you should be able to get a fastpass for one "main ride" and still ride your next choice with barely a wait! Its later in the day it becomes more of a pain!