What is the best chlorine for above ground pool?

There are several types of Chlorine. All have good points and bad points. To answer your question: I recommend Tri-Chlore in a floater or Feeder. These are the 3" pucks or 1" tablets . They are the most economical. 99% active ingredient. Tri-Chlore is Stabilized, meaning it is protected from UV light and will stay in the pool longer. They will leave a white spot in the liner if they come in contact for any extended period of time and they will lower the pH a lot. Cal-Hypo is good but you have to add it every day and there is a lot of residue left over and it can bleach your liner as well. And is not stabilized the sun will lower the level. Di-Chlore is about the same. Sodium Hypochlorite ( Liquid) has several draw backs. One it ruins a lot of clothes, Two is will raise the total De-solved Solids and if they get to high you have to exchange water to lower the TDS. It also will raise your pH. It's not stabilized, The sun will take it out of the pool. Keep in mind you should never put chlorine in through the skimmer if you have a heater or Salt-Chlorine generator as this will eat out the metals. Then you end up with a lot of copper in your pool, and your water locks up. We'll save that for another time.