From my viewpoint the main people right now are Allen, Rudy, Pataki and Romney.
What do they have in common? All are appealing to the Republican base to the extent they can.
Frist, Hagel and McCain are clearly not interested in the concerns of the base. They'll compete for the small group of Dems & Indy's that cross over, and Rockefellar Republicans.
There are a few others making noice about entering, Pawlenty for one, but it's still pretty light news from their quarter.
Of the four first mentioned, Allen is currently the conservatives first choice bringing executive and senatorial experience. Rudy brings 9-11 and strength with WOT and U.N. Romney has to be given credit for his stance on some social issues in Mass of all places and his running of the Olympics, Pataki isn't as ntoiceable as the other three but he's clearly tring to appeal to social conservatives and took Hillary on recently.
None are perfect in every arena, but no candidate ever is and I respect these four for trying to reach out to US. Anyone that doesn't is dead in the water.