One of of the first things he'd think of is to make sure that it stays a secret. There's no one he trusts with secrets like Butters. But the downside is, I'm not sure the notion of getting TOPPED by BUTTERS would sit well with Eric. Butters is a huge wuss and to get topped by someone like him is disgraceful. And let's not forget that Butters is a DUMBASS, while he wouldn't purposefully rat out Eric and would probably be a good sport about the whole thing, there's a chance he could spill the beans accidentally or if someone pressured him enough. So I don't think Butters is the answer.
Now we naturally don't include any dickheads like Craig and those guys, so the only ones left are his closest friends - Kinny, Stan and Kyal.
So Kenny- well, Kenny's poor, I think Cartman would be squeamish about touching his dirty dick. That's the end of that.
Stan is a very tempting choice. You see, believe it or not but Stan is actually Cartman's favorite friend. Cartman is chill with Stan and he seems to enjoy the rare times they get to hang. The problem with Stan is that he probably wouldn't keep his mouth shut. Stan is very sensitive deep down and having sex with fatass would result in him getting worked up about it, he wouldn't be able to forget it (much like his dad Randy acted in that one episode where he jacked off together with Gerald). Stan would treat it like too big of a deal and would probably go tell his little boyfriend Kyle about it, and then it's all ruined. So no, Stan's not it.