Given that Go Set A Watchman takes place after To Kill A Mockingbird, we would think that we should read it second. However, Go Set A Watchman was probably a first draft of To Kill A Mockingbird. I am not really sure in which order they should be read given these two pieces of conflicting information. Which of these should be read first?
Definitely read To Kill a Mockingbird first. Although Go Set a Watchman reads more as a first draft of To Kill a Mockingbird, without reading Mockingbird, much of the bigotry Atticus displays in Watchman will not be unusual. In Mockingbird, Atticus is portrayed as a hero defending the underprivileged blacks, and in Watchman, that illusion of him is completely shattered.
They're both great reads, but reading To Kill a Mockingbird first allows you to have a greater conceptual understanding and appreciation for Go Set a Watchman.