The Kindle's dictionaries (for English, French, German, Italian, and Spanish, at least) are unquestionably better than those of Kobo, Sony, and nook, both in terms of the number of words included, and the quality of the definitions. In each case, the Kindle uses what may be regarded as the "benchmark" dictionaries for those languages.
The thread I've linked to below discusses dictionaries in detail:
https://www.mobileread.com/forums/sho...d.php?t=198435