I think judging from Kate's higher popularity among the A&O fanbase, Humphrey would be the one to be killed off.

I think Humphrey is a little underrated.. I mean he has a personality, even more than Kate in terms of his background story, and how expressive he is.
Kate while being a bit responsible, she isn't as expressive and doesn't have that much of a background story.
(I could be wrong about Kate though)

Also I do not want to depend on popularity since I don't think it counts. XD
Though if I had to pick ANY wolf, that would probably be one of the wolves outside the main four characters.

not winner just like the two

Both are well loved, so if any of them were to die, it would be emotional. But seeing how Humphrey risked his life to save Kate in the first movie and then doing it again in Family Vacation with the trappers... Humphrey will be the smart choice here. Not only will it affect Kate emotional, but show how much Humphrey really loves her, even to the point of protecting her.

Plus, the filmmakers are still ignoring Humphrey's past... so if they were to kill him off and then reveal his past... it will be a touching farewell to our favorite Omega