请问一句介词的使用?语法上讲,Which is the biggest city of the world?是错的,而

of 主要体现的是 前后的从属关系
in 主要体现的是 前后的位置关系

in the world
there is no grammar at this point. its just how we use this phrase.

in (表示位置)在…里面; 在, 于; 在…部位上
of (表示方位)在; 位于 在南部啦北部啦等等
所以是不一样的

of 是属于的意思
maybe 如果infinity is right
but i think you could say both phrases, but they dont have the same meaning.
btw, this sentence is wrong. you might say "Which city is the biggest city in the world?" or "xxx is where im living which is the biggest city in the world."