Each of them have their flaws.
Kenny is overall the most consistent and upstanding of the main 4.Does the least bad of the group anyway and is Mysterion.
Stan and Kyle are interesting to consider.
Stan’s does bad more often, but Kyle has done worse. Pre Canada Nuke and Kyle would be more moral than Stan. Post nuke and its Stan who is better.
Cartman is the worst morally.
Bonus round is Butters who is up there with Lenny, arguably better in some cases.
While it works either way, I’d switch stan and randy.
Stan tends to go after clout and wants to be seen a certain way a bit more randy, who is a bit more greedy and opportunistic than Stan. They both share those traits, though.