Many claim the German language is one of the most challenging languages to learn- more so than French or Spanish. I think that is generally true, but it depends on whether your native language is rooted in Latin or whether it is a Germanic language. Obviously, the more similar the foreign is to your mother tongue, the easier it is for you to learn it. Be that as it may, most would single out German grammar as the main reason why the German language is difficult to learn. While that might be true at least with regard to some topics, German grammar is quite consistent and logical- more so than French or Spanish grammar. So while there are a lot of rules to learn, they usually apply to the large majority of situations, with fewer exceptions than in the Latin-based languages.
Let's consider some examples. As far as word order in German sentences is concerned, the most important rule is that the conjugated verb is second in a main clause, while it is being sent to the end in a subordinate clause. This rule indeed applies to the large majority of all sentences in the language with only few connectors that blur the lines between main and subordinate clauses (e.g. aber, denn, und, sondern, oder), that do not have a conjugated verb at the end (e.g. um...zu, ohne...zu, statt...zu), or that actually connect two main clauses (dann, danach, deshalb etc.). Once students learn these connectors, the theory of German word order is not that problematic anymore. Now in French and Spanish, the word order is very similar to English (subject+verb+other information, unless it is a question where the verb goes first), making the two languages easier with regard to syntax. However, adjectives and pronouns make the word order in French more complicated and messy, While adjectives for beauty, age, goodness, numbers, sizes go before the noun, others go after. E.g. J'aime les robes vertes. J'aime les nouvelle robes. The German language treats all adjectives in the same way. When the French language uses two vous right next to each other, they each have different meaning. E.g. vous vous aimez, the first vous means you (being the subject), and the second means yourself (being the direct object), but the verb comes third.
The second example- the conjugation of verbs and the use of verb tenses- is actually easier in German than in French and Spanish. To begin with, whereas French and Spanish have several different infinitives of verbs that each conjugate differently and with many exceptions, 99% of German words end on -en so there is just one infinitive. Of course, there are regular and irregular verbs in German as well, their endings are usually the same, while their stem changes, and in the plural there is no difference between regular and irregular verbs, with the only exception being our verb for to be (sein). Moreover, French and Spanish have five past tenses with many exceptions to the rules, German has only three (the Perfekt, the Präteritum, and the Plusquamperfekt). Finally, the subjunctive mood in both French and and especially in Spanish (as there are more conjunctions to learn) is quite confusing. Not only is the subjunctive mood used to express necessities, possibilities, and judgments, it is also used in subordinate clauses to communicate subjectivity, uncertainty and unreal situations from the perspective of the speaker. In German, the situations in which the subjunctive is used are more straightforward and similar to English (hypothetical situations, polite questions, advice, suggestions etc.).
The third example is the German cases (nominative, accusative, dative, and genitive) and three genders in the German language (masculine, feminine, neutrum). While the logic of the cases is actually very similar in the three languages, French and Spanish express the difference between a direct and indirect object and the genitive case by virtue of prepositions and their endings, whereas German uses its articles. However, as German has a third gender- the neutral- that the two Romance languages don't have, there are more articles to learn, making the language more difficult in that respect.
Of course, the list of grammar rules students need to master in German and in the other two languages is a lot longer. From adjective endings to numerous prepositions, but what about German vocabulary? It is far from straightforward either. Its words are more nuanced and context-specific than French and Spanish, which tend to be more similar in their use to English, and can rarely be translated literally from English into German and vice versa. On the other hand, many German nouns are compounds of two or more individual words, which is why our nouns tend to be so long. So when putting them together, the compound noun normally receives a very clear new meaning.
Let's finally consider pronunciation. Once you have learned some alphabet differences between German and your native language and remember the sound of some letter combinations like ie/ei, the pronunciation of German words is not as difficult as especially French but to some extent also Spanish. There are rules, of course, but I'm sure everyone who has ever learned French would agree that the language bends its pronunciation rules quite a lot.
So while there might be many rules to learn in German grammar and some might be more complicated than in French and Spanish, that does not mean that all aspects of the German language are more difficult to learn. In fact, the logic and consistency of its rules is what most of my students really appreciate about the German language. Moreover, German is a rich language that gives you access to a new cultural universe- only consider classical music, literature and philosophy- making it quite a rewarding language to learn. So I think the real question is not which language is more difficult to learn, but is learning the language worth your time, given your interests and goals. And that question requires a personal response.
On our blog, you will find many other topics to do with language learning in general and learning German in particular. For instance, we review the online dictionaries dict.cc, dict.leo, Linguee and Collins, discuss the advantages and disadvantages of using Duolingo, Memrise, Babbel, Busuu, and Quizlet, explain the difference between levels of language competence from A1 to C2, and answer the question how long it takes to learn German.