Ok, I knew that title would get your attention, but you will probably be disappointed by my answer. The degree of accent can range from mild to severe in any language. Recently, a gentleman I was working with thought that Indians might have the worst accents — he is from India, and I mentioned that I work with many Indians. I assured him that Indian accents are no “worse” than any others; the fact is simply that there are many Indian speakers where I live, so I end up seeing a lot of Indian accents. Most of the accents I see are moderate to heavy, but I have also worked with milder Indian accents, with several individuals who wanted to eliminate their accent.
With just about every language, I can point to some aspects of many accents that really cause problems with intelligibility. For example, Chinese speakers drop a lot of final consonants, making someone with a moderate or heavy accent very difficult to understand. But the same is true for many Japanese, Korean, and other Asian languages. And Spanish. Yes, Spanish. One of the heaviest accents I ever encountered was from an individual from Venezuela who was dropping a LOT of sounds from the ends of words.
All that being said, I will say that, in my personal opinion, the client I have encountered with the accent most incomprehensible to me was from … Nepal. That’s right, Nepal. This individual was substituting /p/ for /f/; for example, “four” was “pour,” “office” was “oppice.” Super confusing. Yep. Not a sound substitution that we’re used to hearing in these parts.