I would have to say Asian elephant (second only to Common chimpanzee).
I find both species fascinating because their complex social structures make viewing social groupings of them highly engaging. On my visits to Melbourne Zoo in 2023, I spent considerable time watching the interactions of their multigenerational Asian elephant herd; and seeing Taronga Zoo’s chimpanzee community (comprised of multiple families) was similarly a highlight.
It’s no secret elephants are highly intelligent animals, but even then, they never cease to amaze me.
Here is one of many examples (second paragraph) - in the words of Taronga Zoo’s director:
While their trip to South Australia will be a very long road trip, Mr Kerr said Tang Mo and Pak Boon had previously gained a reputation as jet-setters, having flown in from Thailand in 2006.
"Just to give you an indication of how smart these elephants are, after the very first take off and landing on Cocos Keeling Island, when they took off again when they heard the engines start up they knew to lean forward — that's incredible."