More important than decind when to wait in line and when not to is having a plan of what to do when you ARE stuck in a slow-moving line at a themepark. It's wise to bring something to entertain or relax yourself when waiting to board something, as this IS your vacation and every minute you spend on property is very expensive. Bringing along people you like to talk to is the best idea, as any vacation is more enjoyable when you spend it with the people you like. Water is also very nice to have, as a lot of longer queue lines, like Splash Mountain or Test Track, for example, have extended outdoor areas with little shade, and those waterfountains are always too far apart. You can also bring along things like small books and gameboys, but I'm not a fan of having to keep a pack on me.
So, which rides are worth this kind of wait in my opinion?
Test Track
Kilimajaro Safaris (If you even go to Animal Kingdom at all)
Splash Mountain
Space Mountain
Rock n Rollercoaster