Man this can be a tough one to answer depending on what you definitely "greatest" as.

Like for movies you can make the argument that "A Bug's Life" (1998) pioneered future 3D releases featuring animated anthro creatures and put Pixar into such a successful spot for everything that came after. I mean they've gone on to release scores of movies featuring anthropomorphic whatnots, animal or otherwise, and I feel like that helped push the content more into the mainstream for more modern people.

Going back further and into a different medium, 2D movie animation featuring anthros is pretty common (looking at you Don Bluth and Disney) but I feel like "The Lion King" (1994) was probably the biggest success that had really widespread approval and led to tons of fan content and community engagement.

TV shows is a whole other ballpark but I feel like staple series like "SpongeBob SquarePants" (1999) and TMNT (1984) again were standout releases that really popularized anthro characters.

Personally for me I feel like "Zootopia" (2016) or "Zootopia 2" (2025) is probably the greatest anthro movie for 3D animation, "The Lion King" (1994) for traditional, and if you're going into something like stop-motion I'd recommend either "Fantastic Mr. Fox" (2009) or something goofy like the Wallace & Gromit movie where the turns into a rabbit.