Translation:Which language's grammar is the most difficult?
Yeah noun cases is not the only or best measuring stick. In agglutinative languages with many cases, they are close to being like postpositions but glued on. Georgian has postpositions and cases, and sometimes uses the same thing for both jobs with different nuances, and it also has prefixes and circumfixes! And in Georgian the nouns are the easy part and the verbs are scary difficult.
I believe I've heard something about Finnish being one of, if not the hardest language to learn (in terms of conjugations).
Spanish has a few different cases depending on who is doing a verb and when the verb happened, is happening, or will happen, or would've happened; poder, puedo, podemos, pedír, pedíamos, etc
German has four different cases: Nominativ, Akkusativ, Genativ, und Dativ
But Finnish has fourteen different cases, including where an object is positioned. If a word is conjugated differently for where an object is located, then I'd say that's pretty difficult.
Arabic is absolutely the most difficult. The Arabic grammar is summarized in a poem composed of 1,000 verses, each verse talks about one or more grammar rules, and each verse, while talking about the rule, also provides an example of this rule. It is called "Alpheyat Ebn Malek", "ألفية ابن مالك", https://en.wikipedia.org/wiki/Al-Alfiyya_of_Ibn_Malik, https://upload.wikimedia.org/wikisource/ar/b/b4/%D8%A3%D9%84%D9%81%D9%8A%D8%A9_%D8%A7%D8%A8%D9%86_%D9%85%D8%A7%D9%84%D9%83.pdf
Each word in the Arabic language is essentially sourced from a three-letter word (radix) representing the past tense of a verb (all past tenses of verbs are composed of three letters), and the sourcing follows a strict set of rules. This means that words are not generated randomly, but rather there is a strict rule for the composition of words.
The most difficult thing in Arabic is dissecting the sentences, explaining where position of each word (e.g. subject, object, ...); This is a science on its own, and is extremely difficult even for native speakers. The pronunciation of each word (intonation) depends on its position within the sentence; that is, to pronounce Arabic properly, you would need to dissect the sentence while speaking. This comes naturally for native speakers, but is almost impossible for foreigners.
To further complicate the language, different Arab countries have different spoken languages, that can be fundamentally different from the classical Arabic (which I am talking about here). These spoken languages not only present differences in the pronunciation, but also present a different vocabulary and structure. The Egyptian language (fundamentally different from the classical Arabic) is the one most understood by all Arab natives because of the pervasiveness of the Egyptian movie industry.
As someone who was born in Egypt, and can speak a few other languages, I can tell you that Arabic has, hands down, the most difficult grammar.