I took the ACT for the first time as a junior in December and just received my score of a 36. My GPA is only about a 3.9 unweighted and a 4.2-4.3 weighted, which is not as high as you would expect of someone with a 36 ACT. I scored a 2160 on the SAT, which is disproportionate to my ACT. I've been hearing from people who have higher GPAs and SATs than me are getting rejected or deferred from some of the schools that I am looking at. How much weight does a 36 ACT carry?
A 36 ACT score, particularly in combination with a relatively high GPA, can have a powerful effect on your chances of college admission. Most colleges don't want to see both ACT and SAT scores (for exceptions to this rule, read these articles), so you're free to submit the higher of the two - in your case, that would definitely be your ACT score. Unless you're applying to schools like Georgetown or Stanford, your SAT score should have zero impact on your admissions prospects (since you do not have to submit it), so comparing yourself to students with similar SAT scores and GPAs is not productive.
The effect that a 36 ACT score has depends on which schools you're applying to. The lower the school's average ACT score, the greater your chances of acceptance with a 36 ACT score. If you're applying to schools where the middle 50% of all entering students have an ACT composite score of, say, 26, then a 36 (in combination with a 3.9 GPA) makes it very likely that you'll get in. Even if you're applying to a more competitive school with an average ACT score of 33, a 36 will still have some impact, just because so few students manage to get a 36 (less than .1%, compared to the 1% of students who manage a 33). The positive effect of a 36 on the ACT, however, will be magnified if you apply to schools with an average ACT score of 30 or below.
PrepScholar also has a great tool you can use to determine your chances of getting into a school with a given GPA and ACT score. Simply search "prepscholar [school name] act" and you'll find the page. For example, our page for Wellesley indicates that with a 3.9 GPA and an ACT score of 36, you'd have about a 77% chance of acceptance (assuming you met all the other admissions criteria for that school).