Honestly probably the original Ice Age
On the one hand Scrat, Manny, and Diego are likable characters, there's good vocal performances all around, some effective scenes (The cave scene comes to mind), and the underlying threat that these animals are saving someone who could grow up to kill them leads to some good tension. On the other hand there's nothing all that new narratively, Sid isn't funny despite Leguizamo giving it his all, honestly the movie in general is more mildly chuckleworthy than hilarious, and the other saber tooth villains are kind of dull and underwritten.
Also yes that baby isn't cute despite how the movie keeps insisting he is nor has the animation really held up (Some shots look fine while others look nearly Foodfight-esque).
But yeah it's not bad or horrible just nothing that special which is probably why this franchise, despite how many of them there are and how much money they made, never really got the cultural impact that Shrek or any of the Pixar movies or Frozen or Despicable Me or Encanto got, at least as far as the United States is concerned.
However Ice Age 2: The Meltdown is a bad movie. It admittedly has better animation and a more consistent tone but that's all it really has going for it over the original...