It is going to be either Newt Gingrich or Mitt Romney. I believe that Romney has a much better chance than Gingrich.
Gingrich has a lot of flaws that have not been very well pointed out since he has not been anywhere close to being the front-runner until just lately. Romney has flaws, but everyone knows about them and many people have come to terms with them. In addition, Romney simply has a lot more money and a much better organization. Gingrich had not really seemed serious about the race and does not have the infrastructure in place to push him to victory. In addition, as the NY Times link below shows us, the insiders of the Republican Party really do not like him and are working to undermine him. They can't directly kill his candidacy, but they can hurt him and I do not think he has enough of a core of support to weather that kind of an attack.
Essentially all of the "non-Romneys" have had their time in the spotlight and they've all faded. I think Gingrich will too.
Why the other nominee will not win?