Well, maybe not THE most in absolute terms, but this page lists six of the top candidates:6 of the World's Most Intelligent Animals
The only one that surprised me was the pig. While I knew they were smart, I didn't know that in some categories they're thought to rank with dolphins. The other five on the list are the two most obvious, dolphins and chimpanzees, plus ravens, octopuses, and elephants.
Each paragraph on the page includes links to several other sites offering more information about the particular species and how its intelligence has been studied.
Although not mentioned on that web page, other species that would plausibly evolve sapience might include raccoons and bears. Both of them have the ability to manipulate objects and are already smart enough to defeat with ease many human attempts to keep them out of buildings, vehicles, and containers. Until our family bought better-designed garbage cans some years ago, we had to tie the lids on to protect the trash from raccoons. As for bears, you've probably seen photos or videos of them breaking into houses and cars. Scary! Maybe Yogi and Boo-Boo have a basis in fact.
As usual, this topic makes me wonder whether we'd recognize human-level intelligence in analogous alien species if we met them on distant planets. Their languages might not consist of sounds as ours does. (Maybe sapient octopuses would communicate by changing their colors.) Suppose lack of manipulative appendages, as with dolphins, prevented them from inventing material technologies we would recognize as such? What if, like dolphins and octopuses, they inhabited an environment (e.g., water) difficult for us to explore? Also, an ethical question comes to mind: If we don't exercise sufficient respect toward quasi-intelligent species we already know about on our own planet, will be behave ethically toward creatures we've yet to meet on strange new worlds?
Margaret L. CarterCarter's Crypt