This varies WIDELY - but you can usually predict the following:
It takes most people longer to get to the park than they think it will - so early in the morning crowds are at there lightest. Evening EMH is more crowded than morning EMH - but who cares, it is three hours instead of one! (We found the MK evening EMH to be the most crowded EMH, duh! The day after the MK's EMH, the MK was much lighter, and it was still open quite late.)
Most people head for the closest/newest ride fast so while spaceship Earth has a line in the morning, by evening there is little or no line. In Epcot, Mission Space and Test Track have the longest lines - and they will start in the AM. (Strike that - when it opens, Soarin! will have the longest lines.) It will usually be best to Fastpass these - fastpasses for the newest rides might be gone later in the day, so get them early.
"kiddie" rides are much less crowded in the evening - if your little ones can last that long. (Small World has no line at 10pm)
Dumbo (MK) always gets long lines - which I've never understood. It really isn't worth the wait. Save wait time and go on Alladin's Carpets - same ride mechanism, just different decor. Or go on the rockets - same ride, but cooler, because they are up higher. The same is true of the ride in Dinoland (AK) - same ride there, but the lines can be long for this ride, too. So if you ride one of them during EMH, they're basically all the same. It's a cute ride for kids, but as far as rides go, of all the rides at WDW, it is one of the least unique. Universal has the same ride mechanism at IoA, called "One Fish, Two Fish, Red Fish, Blue Fish."
If you see a show type ride is loading, and there is room for you to enter too - go to it. Otherwise, it may be best to go on something else in the meantime. Playhouse Disney, for example.
Try not to waste your fastpass on a ride with less than a half hour wait (or 45 minutes, if the park is really crowded, and all the lines are 30 minutes long). In addition to the wait boards near the entrance to every park, use your eyes when checking the wait time for a ride. CM's get tired of guests asking, "Is the wait really 45 minutes?" They are pretty good about updating the signs, So try not to ask, but sometimes you can just look at the line, and know that the wait will be shorter or longer than the sign says.
Single rider lines often save LOTS of time, if you can use them. Most of the time your party will be split up, and kids who are just over the height restriction can't go in the single rider group (say, if the height requirement is 44", and your child is 47", they will have to have a parent ride with them.)
Tip: Know how many people are in your group BEFORE you get to the CM for each ride! This will reduce wait times for you and everyone else in the line! Listen to what the CM tells you! If you want your kids to ride in the middle of the seat, organize your group ahead of time (Parent first - child second -other parent third). This will also save headache for many rides.