Sophia, Straighterline, or study.com ?

[Flelm]

If you're still in high school, are any AP classes available to you? They're very valuable for Gen. Ed. credit. Often a single AP class can be 6-8 credits when transferred to a college/university.

TESU is fine, but it's intended for adult learners. If you have no college experience, there's extra hoops to jump through: https://www.tesu.edu/admissions/undergrad-admissions. It's also not the cheapest (except for CompSci like rachel83az said), and not the fastest.

I have no real knowledge or experience with COSC so I'll let someone else answer that.

You can take ACE/NCCRS classes and courses, but they don't count as true college credit until they're transferred into a college that accepts them, after you apply. TESU just revised their ACE guidelines and that left a bunch of people scrambling. If you're just focused on collecting credit, I would look at only Sophia because of its price and ease of use until you make more decisions.