At the moment..i think

i just picked Kyle because of the last episode lol :P

He seems to be with kenny alot-hanging around and stuff... sooo ill just say kenny! >.<

he just doesn't rip on Stan at all...for anything.
He rips on Kyle for being a jew...
He makes fun of Kenny a lot for being poor...

and have you guys noticed that in the episode where old ppl kill others when they are driving and they were playing street hockey that he referred to Kenny as the poor kid, Kyle the jew, and Stan just...Marsh.
I have a hole lot more to prove my point, it'd just take all day. But yeah, out of his three "friends", i'd say he likes Stan the most.

^ I agree with everything you said! If he liked Kenny most why would he rip on him just as much as he does Kyle? We all know he hates Kyle more than anyone. Butters is just his little companion if he needs something done. Meanwhile Stan is the kid he has most respect for. He dubs him 'Marsh', respectively, during play. And when they do play together things are so... normal. And calm. And let's not forget he seemed concerned for Stan's illness in Fun With Veal! 'Thank god we stopped it in time.' *places hand near Stan* AWWW!! I think with them, most people simply don't pay enough attention to what they say and do so they think Cartman and Stan's friendship doesn't exist. I would do anything for more Stan and Cartman moments in the show :)

I say he likes Butters the most. He's the only 1 that never makes fun of Cartman.

Butters is just Cartman's bitch. But Cartman calls Stan his bro! That's pretty important. and it's true. Cartman doesn't rip on Stan for anything. And Cartman even said he likes it a lot more when it's only him and Stan. There were a lot of Stan and Cartman moments. But there are a lot of other characters who like Stan more than his other friends. Cartman has a lot of respect for Stan.
If he liked Butters more, he wouldn't try his hardest to humiliate him and he wouldn't do things to hurt him. I think Butters is the one he has the least respect for. And Kyle...well everyone knows Cartman hates Kyle.

Because he is easily influenced and easily manipulated. Also he is gullible, so he can use him for his mischeviousness

I got a scratch, KENNYYYYY!