It depends on what you mean by "best". Do you mean safest? Or the nicest area to swim?
The thing is, with the Dead Sea, it's not so much a case of swimming, but more of a case of floating, due to the heavy saline content in the water - you literally get so propped up by salt that regular swimming is very difficult.
If you do decide to take a dip, make sure you be careful rubbing your eyes when you get out of the water, as your body will be covered in a slimy, salty residue. If you get this in your eyes, it's going to sting like crazy. about 10 years ago