Humanity is very powerful, our simple unique trait of sapience allowed us to conquer the planet with little to almost no resistance. In the popular science fiction series Planet of the Apes; apes, with the help of disease, are able to defeat humanity once they are given sapience.

Let's assume I want to be able to easily kill humanity using a similar method. I was inspired to use the method of giving a different species sapience by Planet of the Apes. But first I must choose my species. What species, given sapience, has the best odds of defeating humanity.

Two points must be addressed to avoid over broadness:

* The chosen species does not need to be able to defeat humanity. They
just need to have better odds when compared to other species.
* Since there are literally millions of possible species available lets limit it to vertebrate land dwellers. As they would be best able to gain this sapience.

To address Aify's concerns by Best, I mean that the species must at least stand not only a chance but when compared to other answers, clearly has a higher likely-hood of defeating humanity.