IMO, it's between Rudy and Allen w/Tancredo as a spoiler if he runs.
The reason conservatives like Rudy is because he's tough on Defense no matter media critism, and he doesn't seek to marginalize them. Maybe he did in past? Don't know, but he doesn't now. Whereas McCain gets off on tweaking us every chance he gets it.
I think the Liberal activists in the primaries have to make a choice. If they really want a shot to get a Lib on the ticket, Rudy's their best shot. Conservatives wouldn't be HAPPY about his liberalism, but most will hold their nose and vote on an '08 ticket because he doesn't hate them and he's solid on WOT. If McCain is their representative choice, they are screwed. McCain hates us, we hate him. He loses.
Allen is the default choice for conservatives. He's not the solid charismatic conservative everyone wants, but he's positioning himself as the most tolerable to the conservative base fiscally, socially, to Hawks, to borders. Conservatives are holding out endorsements hoping a stronger chice emerges, but if not, Allen gets the pick and their support. Newt has too much baggage. Romney remains an unknown. Pence screwed himself with his amnesty proposal.
Then we have Tancredo. If he runs. I'd be surprised if he finished first, but depending on the unity of conservatives and Liberals and their selection of choices, he could squeak in as a second or third place finisher at times. Respectful enough finish to influence a change of policy on this issue at the top. Plus, border security is an issue that bridges Liberals and conservatives in the voting public.
I think it's Allen's to lose. Conservatives like Rudy more, but Allen's more right on the issues generally and in the end it's the issues that are going to matter in the primary. If he's lousy as a campaigner, however, that could create an opportunity for Rudy.