it"s the best --(of/for) them all.为什么?

本题有歧义
for
对他们大家来说这是最好的.
of
在这些（选项）之中这是最好的.
for 对.来说 e.g.It's important for you to learn English.
of .怎么怎么样.e.g.It's nice of you to help me.