The short answer? It’s a tie, roughly speaking, according to Live Science.

The finding is based on a language development test, revealing average dogs can learn 165 words (similar to a 2-year-old child), including signals and gestures, and dogs in the top 20 percent in intelligence can learn 250 words.
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The smartest dog? Border Collie.

And if you’re wondering about cats…see Are Dogs Smarter than Cats?