What is the best case for a 13-inch MacBook Air?

I would recommend a padded slip case. I've been using an inexpensive neoprene one I got on Amazon years ago. If I'm using a bag with a padded slot, I don't bother. If I'm using the computer somewhere other than my own desk or table, and I'm suspicious of the surface, I put a piece of paper under the computer.

Do NOT use any kind of case that clips on. They are notorious for doing damage to the hinges and will interfere with cooling.