Pat Buchanan's not going to run again, as much as I wish it were true. He'll almost surely pick another candidate with similar views.
Out of the current crop, the candidate whose views are most similar to Paul's is Tom Tancredo. Tancredo is also part of Paul's Liberty Caucus (and Paul is part of Tancredo's Immigration Reform Caucus).
With Bush and the Democrats now touting Comprehensive Immigration Reform as the Final Solution to traditional America, immigration is now the most important domestic issue for Republicans (and probably a lot of Democrats too). Only Tancredo has a record as being equally strong on this issue as Paul. This is a MAJOR weakness for Giuliani and McCain, and it would be in Ron Paul's best interest to start talking about his own strengths in this area.
The new immigration bill just introduced is going to be Bush's domestic version of Iraq, and McCain was one of the authors of that bill. McCain has pretty much ended any chance he had of getting the nomination by voting to surrender our sovereignty to foreign nationals.
This is a winning issue for Paul among Republicans and with the American people as a whole.
The only other possiblity may be Duncan Hunter who is also very conservative, with a great record on immigration. But all of the other candidates are weak in this area.