What font is the best for writing with a carbon pen?

Font depends on personal preference. Personally, I think running script is the most beautiful and practical, but to write running script well, we should start with basic regular script.

As for English, it is the normal kind of Chinese characters. The key is that you should have a good knowledge of calligraphy, and the words should be of the same size and arranged neatly to look good. You must be patient and practice point by point from the most basic, which is a long-term effort.