District 1 is by far the toughest, hands down. They have the largest number of kids and the best wrestlers.
That's saying quite alot. District 2 is pretty darn tough all the way around, not to mention Greater Gold out of District 4 is about as well rounded club as you can get from what I've seen. Bottom line depends on age and weight. Every District has State Champions, which means at some age and weight that district is the toughest.