It seems to me that obviously the first place the crowd rushes to is Space Mountain when the park opens. From 12 years of experience I found that it is best to sprint down Main Street and make a hard right turn at the Plaza Ice Cream Parlor. Continue your sprint down the walkway and through the Pavilion Noodle Restaurant. Once you reach the entrance to Tomorrowland, you probably will be momentarily stopped by a line of cast members, but don't panic. Once they let you in, sprint immediately to the entrance of Space Mountain and ride. Next, when you get off the ride, get a fastpass and get back in line to ride again for a maximum wait of 20 minutes since there is only one line. After you ride again, spend your time at the Carousel of Progress and the TTA until your time is up. Then ride again. This way you will have ridden Space Mountain 3 times in the first hour of the Magic Kingdom!
(My personal best time from the entrance to the Tomorrowland entrance is 26 seconds, so if you try this, please time yourself to see if you make any improvements!)