Standardized tests, mainly the SAT and the ACT, are by far the best and most logical ways to measure a student's knowledge and should be the most important factor in determining someone's eligibility for college. As far as I know, these tests are the only way colleges can see a quantitative value that is measured the same for every student in the country, in my opinion, a student's GPA means very little when deciding how "smart" they are compared to standardized tests. I know that a common argument against these tests is that some people aren't "Good Test Takers", but most of these people either have a mental health condition, such as anxiety, or a neurological condition, such as ADHD, dyslexia, or dysgraphia, or are just dumb. For people with mental health and neurological conditions, there are ways to get tested and receive extra time or other testing accommodations. A GPA however, is different at almost every single school.

The reason why I think GPA is far less useful than a standardized test is because of how little it can actually say about a person. I know that for my school, college admissions can see a student's grade in each class, their unweighted GPA, their weighted GPA, as well as general information about the school such as average GPA weighted and unweighted, and the courses offered. The name of the course tells very little about how hard it was because every teacher grades differently and assigns different levels of work. At some independent schools, a regular-level class is as hard or harder as an advanced or even AP-level class at an independent school. There is also a much higher percentage of students who are trying to do as well as they can in a private school than in a public school. Because of this a private school and public school might have similar average GPAs because the public school has easier classes but many students don't even show up, and the private school has harder classes but almost every student tries their hardest to get good grades.

Extracurriculars, which are also important to get into college, are also ineffective in my opinion for determining a student's character and whether or not they should be accepted into a college. First of all, there is nothing stopping a student from lying or stretching the truth about their life experiences. I could lie about how many volunteer hours I had, I could make up a scenario that made it harder for me to do well in school, or I could lie about how an event affected my mindset(a family member's death or you could lie completely and say that you had a close friend who died) because there is no way for them to verify these details.

TL;DR: Standardized tests are the best and only way to decide how "smart" a student is or how whether or not they should be admitted into a college because GPAs are almost useless and their essay and extracurriculars might be one big lie.