- answers

That answer is dependent upon your own learning styles, (learning preferences or response sets) academic interests, ability to pay for tuition, personality, and time available to do study and perform quality coursework. Columbia College is a tier 1 online program with extremely affordable classes at about 630 a class for undergraduare courses and not much more for Master's. Harvard Extension has an interesting and rigorous online program which, although more expensive than CC is still very affordable.
The hang ups with CC are that its online classroom platform is a bit dated and difficult to navigate. HE does not provide financial aid until after the first 3 required prerequisite courses are completed, so that may be an obstacle for you.
Ashford University has excellent courses and you will write a lot of 8-10 page "Reflection Papers," which are really research papers, written in APA format. Although CC uses APA format as well AU is especially strict in its grading process on APA, a strong thesis and well thought out arguments; especially strict, so if you like to write or need to learn how to write at a high academic level, AU is a great place. The downsides of AU are they raise their tuition alot, at 3 times this past year, they are becoming less affordable, though at 1011 a course they are still cheaper than WU and some HE courses as well, though not all of them. Also be very careful with their financial aid department, as they are not always fully honest, though they do obey all federal laws governing lending procedures.
I like CC because they have a great reputation and they are dirt cheap. The professors are pretty involved with the classroom and many of the tests (like HE) are proctored. The amount of suggested and madatory reading materials at CC far exceeds WU, AU, or HE, but WU and HE have more vast online libraries. WU provides more in classroom links for more supplemental readings, but CC hammers you with many textbooks you really need to read atleast in part to keep up with the class. I emphasize quality of education, but you may mean something else when you ask about the best online schools to attend.
There are also many traditional Universities offering partial and some full online programs as well like: Cornell, St. Johns, Penn State, and Nassau Community College. Hope this helps, as I took my time to write this for you...

There is no one definitive list of top 10 schools, so what I think would be a better way of gauging the quality of an online school is to look at the reviews that are posted by past and current students.
To help with this here is a link to a list of online school reviews. This list will help you to develop a better understanding of a school's quality in education, student service, and and more.
- related questions
How do I get an extension on my financial aid, I have two years left?
Are there any online programs for makeup artists?
Dual degree BCA/MCA via correspondence
What is the best degree to go for now with the current job market?