As always it comes to personal experience.
I took two years of German in high school and learned a LOT. Maybe I had an awesome teacher, or maybe German was super easy. When I got stationed in Germany, I had no real problem communicating other than the words I simply didn’t know (meaning I had most of the grammar down pat).
But still… I took a year of French and a year of Spanish in high school too (crappy high school; I transferred from a better district where I’d already fullfilled my requisits). Didn’t learn squat. In French, maybe “baguette” and probably not spelt correctly; in Spanish, cerveza and baño which go hand-in-hand, but not much else. Probably the teaching. And me with language-learning experience.
But I know Spanish now. I lived in a Spanish-speaking country for a year, and married one of their gals, and am about to live there for another year (good industry I’m in). And you know what? Maybe it’s the teacher, and maybe it’s living there, and maybe it’s not having a friggin’ choice. But if I think of it logically, and try to be completely neutral, I’d say Spanish is by far the easiest.
The “way it flows” is super-similar to English. The only real challenge is learning how their verbs work. This may be the same in other languages. (really, read the entire link).
Okay, once you have the verbs, remember that languages are word-for-word translations, and practice a little, then speaking Spanish and English is just the same.
German, though, I just “got.” Not a lot of my classmates and fellow soldiers did, though. It’s kind of like reading Shakespeare or talking Yoda. Not hard at all if you’re open minded and remember, as always, that languages just aren’t about substituting one word for another. Verbs, as in Spanish are trickier than in English. Spanish verbs and tenses and usages are more similar, though. Also German has a lot of cases that aren’t used in English all that much any more, especially the genetive (yeah, we have it, but it’s rare). This and other cases change noun forms a lot. One thing about German that’s closer than English is their seperable verbs, that Spanish doesn’t use. I think in English we call them adverbs or something. It’s probably easier for an English speaker to adapt to than a Spanish speaker, though. Things like, “put on the coffee.” In this case the entire verb is “put on” rather than just put. In German it’s the same idea, but not in Spanish – they’d just say “put the coffee,” which seems easier on the surface, but isn’t nearly as descriptive in my English-oriented mind. Works well enough for them, though, I guess.
So… let’s talk usefulness. If you’re in the USA, Spanish wins hands-down. Okay, disclaim this and that and that. I’ll still utter Spanish.