Every language has its difficulties and its treasures. I would say that if learning a language is fun, it is easier to learn, all the more if you have a good teacher.
For English speaking students, a Germanic language, like German, Dutch, Danish, Norwegian or Swedish ought to be the easiest, although French, Spanish, Italian or Portuguese might be an interesting alternative.
The Celtic languages might also be a "discovery" for amateurs.
No language is really easy, even one's own language.