@jikodesu I'd say, Australian English. They have a similar, if not the same, accent with British / Scottish English.
Though, I noticed, some of them can make adjustments on the fly which makes it more understandable.
Singlish. I guess I've gotten used to it since I used to join #TeamSpeak sessions back in my active gaming days a decade or so ago.
Malaysian English. I haven't met, or talked to one who speaks pure or natural Malaysian accent. At least those I talked to via TeamSpeak, and personally, knew how to adjust.
Filipino English. I'll definitely ask anyone who will say Filipino English accent is hard to understand. Hahaha. Even the Koreans and Japanese who studied Philippine English in the Philippines are easy to understand as compared to their countrymen who learned English on their respective country.