I like McCain and I think he would be the bet Republican choice too. You are right that conservatives would vote for him over any Dem, but they might not go to the polls at all, which would hurt the bid for the Whitehouse as well as Republican Congressional and Senate candidates running in the same election

I'm a registered Democrat, but I don't necessarily vote a party ticket, and I would vote for McCain under some circumstances:

Obama vs. McCain --- vote for Obama
Obama vs. Romney --- vote for Obama
Clinton vs. McCain --- vote for McCain
Clinton vs. Romney --- vote for Clinton