What you are discussing is called linguistic topology and it is far from estalished fact that Spanish is the easiest. I studied the topic but from the perspective of Arabic language instruction for native speakers of English and other languages.
It might be up there, but I have also heard things like Indonesian, which has the simplest grammatical structure. Chinese, speaking of morpho-syntax, is some ways as easy or difficult as English (tense, gender, and number are not more difficult than English, in my opinion, having studied Arabic to fluency and Chinese at the beginner level).
To get back on topic, topologies like this are good for focusing on which specific constructs will cause difficulty, but which is easiest to learn.
It might be up there, but I have also heard things like Indonesian, which has the simplest grammatical structure. Chinese, speaking of morpho-syntax, is some ways as easy or difficult as English (tense, gender, and number are not more difficult than English, in my opinion, having studied Arabic to fluency and Chinese at the beginner level).
To get back on topic, topologies like this are good for focusing on which specific constructs will cause difficulty, but which is easiest to learn.