McCain is, by no means, a moderate. He just doesn't fall into everything the party leadership believes.
He belongs in the GOP. Chafee, Snowe, et al do not.
Regardless of party leadership, on the issues, he's more moderate on the right then most senators.
He's pro-life but doesn't want roe vs wade overturned, he's a "moderate supporter of gun control" (this is debatable, I think he's pretty strong in support of gun control).
He claims he generally leans on the side of deregulation, but he's often in the news in favor of some kind of regulation.
On Ideology, he's short of being a true blue conservative, but he's not a centerist. He's a moderate, on the right of center mold. He's further right then Chafee and Snowe.
Side note: Chafee is now in the news as talking about Reid trying to get him to cross and him declining.