I took four semesters of high school Spanish, then I took five semesters of high school French. When I took my college placement tests, I scored much higher in Spanish, even though French was fresher in my mind.
I know. I mean, it’s easy to figure out what it means, but I’ve never heard the term before.
Of Spanish, Mandarin, and Japanese, Spanish was easiest (I had studied quite a bit of Latin for medical terminology, which helped out, plus I had a crush on a Mexican girl). Mandarin not too terribly difficult once I grokked the notion of tones. The grammar is pretty straightforward. Japanese difficult but doable.
And the fact that I spoke fluently in German, AND that I never did learn French in Toronto, AND that I have even forgotten some Spanish…AND that sometimes I come across English words that I probably should be familiar with but I’m not, is all making me feel like a loser right now….sigh.
I studied German for years, in high school and uni. Then Thai. German was easier, possibly because of that thing about the second foreign language being harder, possibly because German is structured like English to a large extent.
Well, Dutch is just German with an accent, innit? 
Texas born, CA and TX raised. Spanish was easy for me, except for some of the grammar rules. I’ve learned some of Tagalog, French, Russian, and German. Learning Hindi now. Hindi is surprisingly intuitive for me. The alphabet is coming slower for me than for my wife, but I am picking up the speaking of it faster than she is.
I used to think that I could learn any language I wanted if I just put my mind to it. Vietnamese and Mandarin showed me different. I had a hard time with the mouth, tongue, and lip shapes needed for certain words.
Language learning is fun!
French, German and Latin.
French was the easiest for me, German the hardest. I think that has less to do with the innate difficulty of the languages than with the quality of the teachers I had. My French teacher (in High School), while not a native speaker was a brilliant teacher. The less said about the German teachers I had, the better.
I found Latin easier than German but more difficult than French but I think my age (50+) had a bit do with with that.
I went to a school where the prerequisite to studying any foreign language was one semester studying Latin. Want to take French? Study Latin. Want to take Russian? Study Latin. Etc.
Now I lived at the time in a place where most people spoke English and could get by in Spanish, so I knew a few things to say in Spanish, but I’m not going to count it, because it wasn’t actual study. But anyway. I found French a lot easier than Latin (even the first semester of it), and I then found Russian even easier than French, despite the different alphabet and hardly any cognates.
I actually think learning a bit of Latin first helped. Of course our Latin teacher was extremely disappointed that most people only took that first semester. In fact she was bitter and kept lobbying the dept. of ed. to require a whole year, and also to get rid of Russian because it was a commie language and the Russian teacher was herself a commie.
Italian was by far the easiest for me. Spanish too, which I haven’t exactly *formally *studied but picked up by osmosis. Malay and Indonesian I picked up probably more quickly than any other language. Seriously, try Malay, it’s a breeze.
Persian is deceptively easy, in that grasping the basics of the grammar is very simple and straightforward—Persian grammar is comparable to Italian or Spanish, but hugely simplified—but the whammy is when you find that learning how to use the language well is infinitely subtle and tricky.
Hardest damn languages I’ve ever tried: Navajo and Pashto.
Also, you can learn Swahili very quickly if you keep handy a reference source with tables of the noun classes and verbal-pronominal structures to refer to as needed until you get them all down. I taught myself Swahili in one week.
Spanish. Mostly because a) French is my mother tongue and b) I’d already taken Latin before too. So apart from getting over the initial grammar/conjugation hurdle ; and the words they borrowed from Arabic and/or Catalan it was pretty much “speak French, add the odd -a or -o here and there” 
German was probably the hardest - declensions meant having to actually understand grammar which I’ve always sucked at and up to then worked well enough but on purely instinctual level, and vocab-wise apart from a few cheats from English I was lost at sea (and false friends, too… I remember always forgetting that Gift means poison *auf Deutsch. *So, quick tip for y’all : Blutvergiftung ? Not a blood drive. Like, at all.).
Add gendered words that don’t share the gender of their Romance equivalent, and… well, my German was evidently comprehensible, but semantically pretty random
Spanish, Russian, German. German was easiest.
Native Spanish speaker, from Basque-bilingual area, Catalan-speaking maternal family. Have studied more or less formally English, German, French, Catalan and Latin, in descending order of years of classwork (12, 3, 2, 1 and 1).
Romance languages are easier, even French with its quirky spelling rules - the grammar is much more natural to me than Germanic languages. I doubt I’d ever bother take lessons in Italian, after less than a month working in Italy I was able to have conversations in which I was actually speaking Italian rather than the Itagnolo creole.
ETA: nice to see that the French method to “maybe this will be Spanish” is the exact inversion of our “uhm… how do you say this in French… ok, pick the word with the Latin root, cut off the last vowel if there’s one and stress it at the end”.
Absolutely! Though German grammar doesn’t come naturally from speaking Dutch. And our word for horny is their word for cool, which will never stop being awkward. Weirdos going around calling everything in sight horny! 
But in the same way, Portuguese is easy if you already speak French (which I did) and I think overall Portuguese is simpler, so it makes it hard to judge objectively. And then learning Spanish after knowing Portuguese and lots of other languages was so easy it wasn’t really learning, more like… remembering the few differences.
I’ve studied German and Italian, in that order. German is not as hard as some people make it out to be, but Italian is still probably easier, if for no other reason than that it has only two genders instead of three, and most of the time the gender is discernible, whereas in German it’s just anyone’s guess.
I had the six month Portuguese and French courses (in that order) at the Foreign Service Institute, which uses native speakers as instructors. French was the easier of the two for me. I studied Spanish just as a lark, going in the evenings to a local school that also had native speakers. It was the easiest of the three.
I studied French in middle and high school, and had a relative around during that time who was fluent and who helped me practice.
I studied Spanish in college and on-and-off as an adult, because I work with a Spanish speaking population and I like to travel to Spanish speaking countries. I’m not very good, but I can manage a bit. As far as work goes, we have people who are fluent in Spanish, so I don’t use it much in a work context, but at least I can explain to the patrons that a spanish speaker will be over shortly 
I find Spanish easier, both because I learned a bit of another romance language first and because the pronunciation is more intuitive. I have a really, really hard time with accents and pronunciation. I’m also tone deaf and musically illiterate, which probably relates.
Interestingly, I still sometimes think of the french word first. Somehow, certain pieces of French vocabulary have stuck in my brain to the extent that I can’t quite call upon the Spanish equivalents. I’ll start trying to conjugate a french verb with the Spanish endings, with bad gringo pronunciation. It’s a mess.
I tried French, Hawaiian, and Korean. My college had a 1 year foreign language requirement, and in grad school I had to take another year. I failed Hawaiian once and Korean 6 times. The French teacher was a nice guy and passed me so I could get my BA. In grad school, I passed Korean on my first try, and I was the top student in the class who had not spoken Korean previously.
Imho, knowing what I know now, I believe I could be successful learning any language. The key is to think about how children learn language and do it their way: practice, repetition, and no translation. Everytime the teacher said a new word, I repeated it softly. The students sitting around me started copying me. When I studied vocabulary, I studied the word, not the letters. I always went verbally first, written second. When I had a choice between verbal and writing, I always went verbal.
If you are just looking at the easiest language for English speakers, the research says any language with a similar alphabet is easier, e.g. French, Spanish, German, etc. Any language with a new alphabet is harder, e.g. Chinese, Korean, Japanese, etc.
Each language is easier than the previous.
I grew up bilingual (English and Spanish) and added over time German, Russian, and Chinese to varying degrees in that order. Chinese has been the easiest. That’s relative though, I actually don’t learn languages particularly well I think.