根据语法规定和能够作表语的成分来看，the best way 的表语可以是不定式、动名词、从句、名词甚至介词短语等。例如：

“（了解一个人）最好的办法是观察他的言行。”

The best way  (to understand a man) is to observe his words and actions.

The best way  (to understand a man) is observing his words and actions.

The best way  (to understand a man) is that you observe his words and actions.

The best way  (to understand a man) is the observaton of his words and actions.

The best way  (to understand a man) is by/through observing his words and actions.

最常见最常用的表达，当属动词不定式。

以上几种表达方式，可以开阔思路，表语成分不要局限于不定式和动名词，在现实语言中做到灵活运用。关于使用不定式，还是动名词以及它们的区别，意义不大。学英语应学会表达多样化，才是最重要的。