GCIDE might be the best one of the free dictionaries, it is Webster's 1913 with modern additions. But Kindle can still use this:
https://www.amazon.com/Collins-Engli...dp/B00JO2Y7P4/
If you care about the dictionary on your ereader...
It is just too bad that Kobo only gives us The Merriam-Webster Dictionary. They need to license something better.