For an English speaker learning a first foreign language, per the American Foreign Service Institute (AFSI). those languages require these many hours of study to reach competency (not fluency, which always takes much longer):
level I: 575-600 hours. French, Italian, & Spanish. They all evolved from Latin and have more commonalities than differences.
Spanish - largest number of native speakers, most of whom are in the Americas and Spain. Spanish is easier than French in beginning, but it balances out if you learn to competency. If you don't learn to competency, then Spanish or Italian are your better choices, if difficulty is your only concern.
Italian -- not that many native speakers and not that many that learn it as a foreign language - to competency. It is also easer than French in beginning, but balances out at advanced levels.
French - has native speakers in Canada, some countries in Central/South America, France, Belgium, some places in the Pacific Islands, and a lot of places in Africa. It has the largest number of competent speakers as a foreign language, second only to English. You are likely to find competent speakers almost all around the world, even in Asia.
Spanish/Italian are far more phonetic than French or English. Unlike English, French rules have very few exceptions. However, the rules are numerous and complex. Surrounding letters often matter, as well as diacritic marks. There are often silent letters as well.
In addition. English, Italian, and Spanish are all stress-timed languages, but the stress patterns vary.
French, however, is mostly syllable-timed (most syllables get equal stress, except for the very last one of a sentence).
French, more so than the others, often requires that sounds be added, removed, changed, and/or moved beyond syllable boundaries. There are also times when such changes are optional or forbidden. Often, the pronunciation of a phrase is different from that of its words in isolation.
All three have two grammatical genders: masculine and feminine (a separate concept from words for physical sex.. English has NO gr. gender, but does have words referring to physical sex). Spanish & Italian both often clearly mark the gender on the word itself. French often does not, and when there are markings, there are usually plenty of exceptions.
The verb systems are more complex, both in number of form and in tense/mood/aspect combinations, than French. The subjunctive mood is one advanced topic that is far more complicated than the French version. Several of the French equivalents of some Spanish/Italian tenses only exist in modern French as literary tenses and aren't needed for the spoken language.
level V (2200 hours of study). Japanese, Korean, and all the Chinese languages (most people learn Mandarin, the official national language of China). Level V languages are considered among the hardest languages for English speakers.
All three are very different from English (or French/Italian/Spanish -- all four of these are Indo-European languages and share many concepts, at least at a high level. Details can differ vastly). They are also different from each other far more than French/Italian/Spanish are from each other. They are all in their own language families.
Chinese - relies almost exclusively on word order. Words don't change form.
However, there are many idiomatic structures. About 4000 Chinese characters are needed for daily literacy (and there are at least tens of thousands more). There are two styles: traditional & simplified.
Chinese languages use tones to differentiate homophones.
Chinese has more speakers than other languages, due to the huge population of China. Mandarin is often just a second language, though, to many Chinese. Most people learn to written in standard Mandarin as well (but other Chinese languages have their own syntax, vocabulary, special structures, and sometimes even characters as well).
Japanese & Korean both mostly rely on agglutination, followed by inflection. Word order is potentially freer than in all the other languages we have discussed. The only big requirement is that verb phrases (or also verbal adjective phrases in Japanese) end the sentence.
Japanese uses about 2100 Chinese characters for daily literacy. Most have at least 2 pronunciations (Japanese adopted a lot of Chinese vocabulary). It also use two syllabic scripts. All three scripts form one complex writing system. Each script has specific uses and all three can appear in even a simple sentence (like: I am American). The two syllabaries are roughly about 100 more symbols together.
Japanese has a simplified type of tonal system called pitch accent, but it varies among different dialects and is often poorly taught to foreigners. Only a few words change meaning with the wrong pitch, but the wrong one produces a very thick foreign accent.
Korean uses its own alphabet. It is far more phonetic than the English alphabet. It has several more letters but it's still only 30 something letters (not thousands).
Standard Korean has no tonal system of any kinds, but some dialects do have pitch accent.
I highly recommend you do at least one level I language before attempting level V, unless you are very strong in spelling, vocabulary, and grammar in English and have strong motivation to learn one of them first instead.
All three have very alien ideas for an Indo-European language speaker (like English).
There are people who will claim that any one of those three is "easy". There is no such things as an easy foreign language. There are only levels of difficulty, and level V are very hard over all. There are many things that seem or are simpler than their English counterparts, but they all have their own difficult concepts as well.