I am torn between Pacific Walrus and Eastern Gorilla - the latter is rarer, but the former was more active and spectacular. I didn't get the best photos of either, sadly. The walrus photo was a great opportunity, as a pair approached the very front of their enclosure to investigate us, but was taken from a phone, while the gorilla photo was taken from a proper camera, but at an inconvenient angle and through reflective glass.
Bolt the King Cheetah at Zoo Negara, who I'm pretty sure is the only one in Southeast Asia. He was conceived whilst in South Africa before his move to Malaysia.
Both of my wild monotremes have definitely been memorable. 10 wild platypus sightings at a few sites now but only three echidna sightings exclusively in Tasmania. No wild echidnas on the mainland yet.