What is the best copybook to practice block letters?

There is no best, only better. A more effective way is to practice writing brush. With a good brush, pen writing is naturally good. Yan, who is famous for his calligraphy, is up to you, everyone. It is generally acknowledged that Ou Yangxun's standard of regular script is very high, and "Jiucheng Palace Monument" is his masterpiece. Yan Zhenqing and Liu Gongquan also have their own characteristics, and there are many copybooks. Zhao Mengfu writes beautifully and has a lot of copybooks. In my personal opinion, the regular script of the two kings and their sons is by no means lower than the top four. Just choose what you like, but don't change jobs casually.