Panda
The giant panda has achieved a level of cultural penetration that most marketing departments can only dream of. It serves as the logo of the World Wildlife Fund, has inspired countless films including the inexplicably successful Kung Fu Panda franchise (which has grossed over $1.8 billion worldwide), and functions as China's primary tool of soft power diplomacy. Pandas are loaned to foreign zoos at rates of approximately $1 million per year, and the birth of a panda in captivity generates international news coverage. The panda has transcended animal to become brand.
Hippo
VERDICT
The panda operates at a level of cultural significance that the sloth simply cannot match. Whilst the sloth has found its niche as an internet curiosity, the panda influences international relations. One is a meme; the other is a diplomatic instrument. The panda's cultural dominance is comprehensive and unlikely to be challenged.