The best correspondence courses will be those that are accredited. The courses must be through a school that is accredited so you know that the school has been examined and has passed muster as an actual school. Find out what schools are offering the courses you wish to take, and then look for the schools' accreditation information. Look up the agency that the school claims to be accredited by to make sure it is a real accreditation industry. You can still take courses from a school that is not accredited, but those courses won't count for anything other than a certificate of completion from the particular school.
The best correspondence courses are the ones that are highly detailed. Look at the course description before you enroll. Make sure that there is information that shows you how detailed the course is and how much information it contains. You want to ensure that you are learning as much as possible. Courses that are a general overview might be good to start with, but the more detailed ones will give you more knowledge about the subject.
The best correspondence courses will be the ones that have credits that are transferable. To figure out which ones these are (they will be different for everyone), look around at some schools you may go to in the future, or the college you attend or plan on attending. Make sure that the school will accept the credits from your correspondence courses. If they won't, you'll get the knowledge but it won't count toward your degree program at all.
The best correspondence courses will be the ones that include communication from an instructor. Courses that only have a sheet of instructions without any instructor information aren't the best because if you don't understand something you won't be able to figure it out. Find courses that include email addresses, websites or online course material including online access to your teacher.