What is the best brand of men's sweatpants? Nike, Adidas, Puma, Russell, Champion, or other?

There is no "best" brand. It's a personal choice. For every 100 people who say that Nike is the best you will find 100 others that swear Puma is the best, or Reebok, or Russell, etc. You need to go to the store and check out each brand on your own and see what you like. Personally, I like Reebok. My son loves Nike. There you have it.