It's interesting to see the non-Robertson (less crazy?) elements of the religious right trying to take [the "Rudy is more electable"] argument by the horns. Almost all the Democrats I know think these people are wrong and Giuliani would be the strongest GOP nominee. It still seems to me, though, that Giuliani is pretty likely to prompt a spoiler candidacy, especially if he somehow manages to win with the 30-35 percent he's currently pulling in the polls.
I'm often inclined to think that Giuliani would be the strongest GOP nominee - because he's a celebrity, a national hero, and a very capable politician. I'm skeptical that he'd be the strongest GOP nominee because he's pro-choice, which is something that his supporters often suggest, and that elite-level Democrats and Republicans alike seem to believe. Not just because of the possibility of a spoiler candidate (though that certainly factors in), but because there's at least as much reason to think that Giuliani's abortion views would hurt the GOP with socially-conservative, economically moderate voters (with the middle-class white Catholics, for instance, who broke for Bush heavily in '04) as there is reason to think that they would help the GOP with socially-liberal fiscal conservatives. And the election map is simply too complicated to say how these effects would balance out.
More generally, I'm on the record as thinking that Rudy's ability to reshape the map is overrated, and that if he is the strongest candidate, his strength will manifest itself in the existing swing states, not in New Jersey or New York or where-have-you.