It depends on an issue of which is the best.

划线部分是 issue 的定语。which is the best 与 issue 是“同位关系”。

of 后面接 wh-word 从句 很常见。issue, question，problem 经常接 of + 从句。

类似例句：

Such arguments ignore the question of where ultimate responsibility lay.

这些争论忽略了最终责任由谁来负的问题。

《柯林斯高阶英汉双解学习词典》

The issue of where it will be held can greatly multiply the amount of time required to reach a settlement. 

案子将在何处受理的问题有可能大大增加达成协议所需的时间。

The problem of where we are to go is still being discussed. 

我们将去哪里的问题仍然在讨论中。