This is difficult because very very few high school students have any idea what they will end up studying. Most of the events that changed my life happened between ages 17 and 22. I changed my major three times before settling.
GPA will always be important because...well, we love to be able to attach numbers to people to guage progress and overall worth. Don't worry though, I got accepted with a 2.9 high school GPA and am now a 4.0 college student. Of coruse, it was a tough sell initially.
ACT and SAT are probably the most important. As others have said, this compares you to others nationally and gives them a good idea of how well you will do.
I would argue that the courses you take in high school matter very litte; probably the lowest on the list. Though they don't hurt. You would do better to get good grades in easy/moderate classes than to get Cs in the difficult ones.
Teacher reccommendations certainly don't hurt. Bear in mind however that the majority of professors feel that about 50% of their recent high school graduates are under-prepared for college coursework. Therefore, there is a major disconnect in opinion between teachers and professors.