General ranking or ranking by subject, what I should take in consideration?

Hi guys.

Yesterday were published the ranking by subjects of the university by QS Ranking.

Well, looking to specific universities, I found out that there are some that even if in the general raking do not have a very high score, in the ranking by subjects have good scores.

An example?
https://www.topuniversities.com/universities/politecnico-di-milano

Here in the general ranking is 187 which is not very high, by in the ranking by subject, for example, in Mechanical engineering is 17th.
And probably universities that is 15th in the general ranking do not have a so high score for mechanical engineering
( it's an example, I do not intend to study mechanical engineering).

So, looking at that, should I consider the general ranking or the ranking by subject?
Should someone prefer a university with a highest score in the general ranking, instead of another one that have a highest score in the ranking by subjects ?

Thanks
WiSi