I definitely think that Appa is cuter than those new ones! What do you guys think? If Appa was real i'd hug him all day long. He's so cute, cuter than those baby bisons i don't know why but im inlove with Appa's cuteness lmao.
None of the others had that. Heck, not even Naga had that, and not only was she based on the original design for the character that would eventually become Appa, she is to Korra what Appa was to Aang...though at least Naga had an absolutely adorable little mini-comic about how she and Korra first met. The closest thing we have with the other bison is the episode Original Airbenders, and they're tangential to the episode itself; Despite being named for the bison, the episode is more about the new Air Nation than it is about them.
So yes, I think Appa is more likable, or rather, more easily likable, than the other bison...but I don't think it's because he's cuter in a physical sense, but rather, because we get to know Appa far better than we do even many human characters in the series.