Here's a brutally honest (Oh boy, am I going to get a lot of angry emails) breakdown of all five of the top tier contenders (Full disclosure: I was a consultant for the Duncan Hunter campaign and he's still my favorite candidate).
John McCain: Johnny Mac is a fiscal conservative, has a lot of experience, particularly on foreign policy, has a strong pro-life voting record, and he polls very well. This is a man who can win over independent voters and therefore, is capable of beating Hillary Clinton.
However, the New York Times’ favorite Republican has made a career out of breaking conservative hearts on everything from campaign finance reform to immigration, to taxes, to judges, to enhanced interrogation techniques. It's a pattern that has been repeated again and again. A huge issue for conservatives comes up and then McCain leads a group of squishy Republicans who very publicly side with the Democrats. In return, McCain gets to bask in waves of positive attention from the mainstream media, which lauds him as a straight-talking maverick in return for his betrayal of conservative principles.
That's why John McCain is loathed by so many grassroots conservatives and it's why he would be extremely frustrating to conservatives if he made it into the White House. Moreover, it's entirely possible that his excellent polling numbers which are based on the great press he receives, would evaporate along with all his positive coverage once he became the GOP nominee -- instead of the Republican in the Senate most likely to side with Democrats. So, even his solid poll numbers that appear to show he’s electable are far from a sure thing.
Mike Huckabee: Despite what some people are claiming, Huckabee would be a fun guy to run against Hillary. That's not just because he would drive the Christian-hating netroots crazy, but because you just have to think that a former pastor who's the most charismatic, likable candidate in the race, would almost have to do well against the cold, corrupt, wicked witch of New York. Trust me; if it all boiled down to Huckabee and Hillary, no one would have a hard time trying to figure out an answer to the classic, "Which candidate would you most like to go to a ballgame with," question. Heck, even Bill would probably choose Huckabee if he thought his wife wouldn't find out about it.