I am French and I spent my days speaking English with people from various nations for the last 25 years.
I heard English spoken in many different ways, some were easy to understand, and some difficult. French pronunciation lays somewhere at the bottom, and I find Germans speaking English very understandable.
Vocabulary and grammar aside, are there "national accents" (with all disclaimers this brings) that are recognized to be "perfectly neutral" for English? In other words, are non-native speakers of English from a particular country recognized as being statistically the easiest to understand for other non-native speakers?